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The nervous systems of Tylodelphys metacercariae (Digenea: Diplostomidae) from the catfish Clarias gariepinus (Clariidae) in freshwater habitats of Tanzania

Published online by Cambridge University Press:  01 December 2015

F.D. Chibwana and
G. Nkwengulila
F.D. Chibwana*
Affiliation:
Department of Zoology and Wildlife Conservation, University of Dar es Salaam, PO Box 35064, Dar es Salaam, Tanzania
G. Nkwengulila
Affiliation:
Department of Zoology and Wildlife Conservation, University of Dar es Salaam, PO Box 35064, Dar es Salaam, Tanzania
*
*E-mail: fredchibwana@udsm.ac.tz; fredchibwana@yahoo.com
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Abstract

The nervous systems of three Tylodelphys metacercariae (T. mashonense, Tylodelphys spp. 1 and 2) co-occurring in the cranial cavity of the catfish, Clarias gariepinus, were examined by the activity of acetylthiocholine iodide (AcThI), with the aim of better understanding the arrangement of sensillae on the body surface and the nerve trunks and commissures, for taxonomic purposes. Enzyme cytochemistry demonstrated a comparable orthogonal arrangement in the three metacercariae: the central nervous system (CNS) consisting of a pair of cerebral ganglia, from which anterior and posterior neuronal pathways arise and inter-link by cross-connectives and commissures. However, the number of transverse nerves was significantly different in the three diplostomid metacercariae: Tylodelphys sp. 1 (30), Tylodelphys sp. 2 (21) and T. mashonense (15). The observed difference in the nervous system of the three metacercariae clearly separates them into three species. These findings suggest that consistent differences in the transverse nerves of digenean metacercariae could enable the differentiation of metacercariae to the species level in the absence of molecular techniques. This, however, might require further testing on a larger number of species of digenean metacercariae.

Information

Type
Research Papers
Information
Journal of Helminthology , Volume 90 , Issue 6 , November 2016, pp. 712 - 718
Copyright
Copyright © Cambridge University Press 2015 

Introduction

Digenetic trematodes are implicated in several important parasitic diseases of humans and animals, such as schistosomiasis, clonorchiasis, opisthorchiasis, fascioliasis and diplostomiasis. This is due to the capability of their developmental stages to adapt to completely different environments or both poikilothermic and homeothermic hosts (Solis-Soto & De Jong-Brink, Reference Solis-Soto and De Jong-Brink1995). Trematodes also produce neurosubstances that enable them to evade the immune activities of the hosts (Duvaux-Miret et al., Reference Duvaux-Miret, Stefano, Smith, Dissous and Capron1992). Since the neurosubstances originate from the host in response to the parasitic stimulus (De Jong-Brink, Reference De Jong-Brink1995), and the release of neurosecretory material accords with the parasite's change from poikilothermic to homeothermic hosts (Gustafsson & Wilkgren, 1981), the study of the whole nervous system would contribute greatly to the knowledge of the phylogeny of trematodes (Grabda-Kazubska & Moczon, Reference Grabda-Kazubska and Moczon1981).

The catfish, Clarias gariepinus, is the host to three Tylodelphys species metacercariae co-existing in the cranial cavities (Musiba & Nkwengulila, Reference Musiba and Nkwengulila2006; Chibwana & Nkwengulila, Reference Chibwana and Nkwengulila2010). Morphologically, the three diplostomid metacercariae are easily separated. Tylodelphys mashonense has a flat, oval body with fore- and hindbody distinct, though not as distinct as in the European congeners, and well-developed pseudosuckers (Beverley-Burton, Reference Beverley-Burton1963). Metacercariae of two other Tylodelphys species, designated as Tylodelphys spp. 1 and 2, are morphologically similar. They have a dorsoventrally flattened forebody and a small, conical and translucent hindbody, with no clear separation between them, and the pseudosuckers are lacking (Chibwana & Nkwengulila, Reference Chibwana and Nkwengulila2010). They can be differentiated based on size and the presence of gonadal enlargen in Tylodelphys sp. 2. Furthermore, molecular analysis of the three morphotypes revealed the presence of three clearly distinct species (Chibwana et al., Reference Chibwana, BlascoCosta, Georgieva, Hosea, Nkwengulila, Scholz and Kostadinova2013). Since the three Tylodelphys metacercariae types co-exist in the same locality within the fish host, it seems interesting to study whether their nervous system structures are similar with respect to the similar function they perform.

Studies of the nervous system of diplostomids are commonplace in Europe; those of Diplostomum pseudospathaceum in particular – its nervous system has been well documented in all stages of the life cycle (Niewiadomska & Moczon, Reference Niewiadomska and Moczon1982, Reference Niewiadomska and Moczon1984, Reference Niewiadomska and Moczon1987, Reference Niewiadomska and Moczon1990). In Africa, on the other hand, knowledge of the nervous system of diplostomid species is missing, with the exception of the partially studied T. mashonense (Beverley-Burton, Reference Beverley-Burton1963). Accordingly, the aim of the present study was to examine the nervous systems of the metacercariae of the three Tylodelphys species occurring in the cranial cavity of C. gariepinus, in order to better understand the biology of these parasites and thus provide information for taxonomic purposes.

Materials and methods

Fish collection and parasite recovery

Fish were bought from artisanal fishermen from the Ruvu River. The metacercariae were recovered from the brain cavity of C. gariepinus by opening the cranial cavity using a strong and sharp scalpel. Metacercariae were removed from their sites in the front of the brain and along the nasal cavity by squirting tap water using a pipette. They were left for 3 h in water in a watch glass in order that the immature ones died and were removed before subsequent treatments (Shigin, 1986, cited in Musiba & Nkwengulila, Reference Musiba and Nkwengulila2006). All subsequent examinations were carried out on mature metacercariae as described below.

Examination of the nervous system

For histochemical investigation of the nervous systems, the metacercariae were placed in physiological saline to which a trace amount of 4% formaldehyde (formic acid and methanol free) was added (0.01 ml per 1 ml saline) in order to paralyse the muscle system, thereby preventing rapid and irreversible contraction of the body caused by the higher concentration of formaldehyde added subsequently. This pre-fixation was controlled visually. Immediately after the relaxation of the muscles, the saline medium was replaced by 4% formaldehyde buffered to pH 7.0 with 0.05 m cacodylate buffer (Niewiadomska & Moczon, Reference Niewiadomska and Moczon1982).

Materials were left in the fixative for about 1 h at room temperature, then washed several times with distilled water to remove all traces of fixative, and incubated in a medium consisting of 20 mg acetylthiocholine iodide (AcThI) dissolved in 1 ml of water and added to 10 ml of stock solution containing 0.3 g copper sulphate (CuSO4), 0.38 g glycerine, 1 g magnesium chloride (MgCl2), 1.75 g maleic acid, 30 ml of 4% aqueous sodium hydroxide (NaOH) and 170 ml of 40% aqueous (saturated) sodium sulphate (Na2SO4). Material was then washed in three changes of 40% Na2SO4, followed by dilute yellow ammonium sulphide (NH4)2S for 2 min, and then thoroughly washed in distilled water. The material was then counterstained in 0.5% aqueous eosin. The counterstained metacercariae were mounted in glycerine jelly and line drawings were made with the help of a camera lucida. Photographs were taken with the aid of a Motic microscope camera with Motic Image Plus 2 software (Motic®, Xiamen, China). Terminologies were adopted from Niewiadomska & Moczon (Reference Niewiadomska and Moczon1982, Reference Niewiadomska and Moczon1984, Reference Niewiadomska and Moczon1987, Reference Niewiadomska and Moczon1990).

Results

The central nervous system (CNS) of the three metacercariae from C. gariepinus consists of a pair of cerebral ganglia (CG), from which anterior and posterior neuronal pathways arise and inter-link by cross-connectives and commissures. The peripheral nervous system (PN) includes innervations of the alimentary tract, reproductive organs, attachment and digestive organs (ventral sucker, oral sucker, pseudosuckers and Brandes organ) and the sub-tegumental muscles. Both the CNS and PN are bi-laterally symmetrical and better developed ventrally than laterally and dorsally. However, cholinesterase enzyme activity (ChE) revealed remarkable anatomical differences of the nervous system among the three metacercariae.

Metacercariae of Tylodelphys spp. 1 and 2

The nervous system of metacercariae of Tylodelphys spp. 1 and 2 (Figs 1, 2 and 3) is composed of a pair of cerebral ganglia, as in most digenetic trematodes, from which two large ventral longitudinal nerve cords (VC) run posteriorly to form a loop around the Brandes organ. The ChE activity also showed two minor dorsal longitudinal nerve cords (DC) and two peripheral longitudinal nerves (PN). The considerably thin dorsal nerve cords are connected to the lateral nerve cords via dorsolateral connectives, while they are linked to the ventral nerve cords via dorsoventral connectives. The longitudinal nerve cords are joined by transverse commissures (TN). However, there are more transverse nerves in Tylodelphys sp. 1 (30) than in Tylodelphys sp. 2 (21). In both species the central transverse nerves (TN) are more concentrated on the ventral sucker region. The ChE activity showed more nerves in the hindbody of Tylodelphys sp. 1 than in the Tylodelphys sp. 2. Both species have caudal ganglia (TG) that coordinate the hind nerve to the nerve loops. The anterior and suprapharyngeal nerves supplying the oral region also vary: in Tylodelphys sp. 2 ChE activity showed a network of nerves supplying the oral region while in Tylodelphys sp. 1 there are nerves from the cerebral ganglia supplying the oral region, as in T. mashonense.

Fig. 1

The nervous system of the metacercaria of Tylodelphys sp. 1 from Clarias gariepinus to show (a) the entire body, (b) the forebody and (c) the hindbody; BN, nerves supplying Brandes organ; CG, cerebral ganglion; CN, caudal ventral nerves; DC, dorsal nerve cord; HB, hindbody; LN, nerves supplying pseudosucker region; NB, Brandes organ nerves; NG, nerve connecting cerebral ganglia; NL, nerve loop; NT, nerves supplying excretory pore; ON, nerves supplying oral region; TG, caudal ganglion; PN, peripheral longitudinal nerve cord; TN, transverse nerve; VC, longitudinal ventral nerve cord; VN, nerves supplying ventral sucker; XN, suprapharyngial nerves.

Fig. 2

The nervous system of the metacercaria of Tylodelphys sp. 2 from Clarias gariepinus to show (a) the entire body, (b) the forebody and (c) the hindbody. NJ, nerves connecting Brandes organ. See Fig. 1 for the key to the rest of the lettering.

Fig. 3

Inverted photographs of entire (a) Tylodelphys sp. 1, (b) Tylodelphys sp. 2 and (c) Tylodelphys mashonense from Clarias gariepinus to show the cephalic and caudal nervous systems. FB, forebody. See Fig. 1 for the key to the rest of the lettering.

Metacercariae of Tylodelphys mashonense

The nervous system of metacercariae of T. mashonense (Figs 3 and 4) is composed of two thick and prominent longitudinal ventral nerve cords (VC) originating from the cerebral ganglia, running posteriorly, one on each side of the body, and joining to form a loop around the Brandes organ. The longitudinal nerve cords are connected by 16 transverse commissures (TN). The ventral nerve cords also communicate with the lateral nerve cords by means of ventrolateral connectives. Two considerably thick nerve cords (LN and ON) arise from the cerebral ganglia and run anteriorly, to innervate the pseudosuckers and the oral region, respectively. The ventral sucker is innervated by nerves branching from the 11th transverse nerve (VN). In the hind body, two considerably thin branches of nerves arise, one from each ventral nerve cord, at the holdfast region, and re-connect at the level of the excretory pore. Two other thin branches originate dorsally from the posterior region of the ventral nerve cord loop (NL) and join the ventral thin cord at the excretory pore. Seven ventral transverse nerves connect the ventral and dorsal nerve cords.

Fig. 4

The nervous system of the metacercaria of Tylodelphys mashonense from Clarias gariepinus to show (a) the entire body, (b) the forebody and (c) the hindbody. See Fig. 1 for the key to the lettering.

Discussion

The application of the s-acetylcholine iodide staining technique was crucial for determining the arrangement of the nervous system of the diplostomid metacercariae infecting the catfish, C. gariepinus. As in other studies on digeneans (see Niewiadomska & Moczon, Reference Niewiadomska and Moczon1982, Reference Niewiadomska and Moczon1984, Reference Niewiadomska and Moczon1987, Reference Niewiadomska and Moczon1990; Halton et al., Reference Halton, Shaw, Maule, Johnston and Fairweather1992; Pax & Bennett, Reference Pax and Bennett1992; Gustafsson et al., Reference Gustafsson, Halton, Kreshchenko, Movsessian, Raikova, Reuter and Terenina2002; Arafa et al., Reference Arafa, El-Naggar, El-Abbassy, Stewart and Halton2007), it was shown to be an ‘orthogon’, a rectilinear, ladder-like configuration of longitudinal nerve cords connected at intervals by transverse ring commissures. The structure and functions of the nervous system in Diplostomum species have been documented in detail by Niewiadomska & Moczon (Reference Niewiadomska and Moczon1982, Reference Niewiadomska and Moczon1984, Reference Niewiadomska and Moczon1987, Reference Niewiadomska and Moczon1990). Niewiadomska & Moczon (Reference Niewiadomska and Moczon1982, Reference Niewiadomska and Moczon1984), studying the nervous system of D. pseudospathaceum, concluded that the structure of the nervous system in the metacercaria was modified from that of the cercaria, which consisted of the cerebral ganglia and three pairs of stems joined by eight commissures, and that it grew with the increasing body dimensions of the metacercaria after penetration of the intermediate host. The nervous system differentiates and forms numerous commissures simultaneously with differentiation of the metacercarial body. This suggests that the observed differences in the nervous system of the three metacercariae Tylodelphys spp. 1 and 2 and T. mashonense described here reflect three systems of three different cercariae yet to be described.

The nervous system of the metacercaria of T. mashonense, as described here, closely resembles that of D. pseudospathaceum studied by Niewiadomska & Moczon (Reference Niewiadomska and Moczon1984). Nerves are more distributed to the vital organs such as the oral and ventral suckers (ventral surface), to enable the metacercariae to exercise the needs of a parasitic life (attachment). The significance of the concentration of nerves on the ventral surface is better explained by the features revealed by chaetotaxy and scanning electron microscopy (SEM), i.e. the papillae on Diplostomum species (Niewiadomska & Moczon, Reference Niewiadomska and Moczon1984; Field & Irwin, Reference Field and Irwin1995; McKeown & Irwin, Reference McKeown and Irwin1995). The SEM of T. mashonense was undertaken by Nkwengulila (Reference Nkwengulila1995), who reported an increased number of papillae on the ventral surface and around vital organs such as the Brandes organ, oral and ventral suckers. The findings of the present study are consistent with Nkwengulila's (Reference Nkwengulila1995) findings. Both chaetotaxy and SEM studies have shown that in Diplostomum species a greater number of sensillae (papillae), which are thought to be tango- or chemoreceptors, occurs on the ventral surface, most likely due to the need to have close contact with the host.

However, Tylodelphys spp. 1 and 2 and T. mashonense show some differences in the number and the compactness of transverse commissures in comparison to the descriptions of D. pseudospathaceum by Niewiadomska & Moczon (Reference Niewiadomska and Moczon1984). In the material under study, the transverse commissures are more compacted, especially in Tylodelphys spp. 1 and 2, and possess a large nerve loop joining the ventral longitudinal cords. It is from this loop that the nerves innervating the hind bodies originate. However, in both T. mashonense (present study) and D. pseudospathaceum (Niewiadomska & Moczon, Reference Niewiadomska and Moczon1984) nerves also innervate the pseudosuckers.

There are too few studies on the nervous system of Diplostomum species and other flatworms in general for one to make comparisons. This has been attributed to the difficulty in studying flatworms (Halton et al., Reference Halton, Shaw, Maule, Johnston and Fairweather1992; Pax & Bennett, Reference Pax and Bennett1992). The small sizes of most parasitic flatworms, the physiological problems of maintaining them in vitro and difficulty in maintaining the histochemical reactions are the reasons for the difficulty in detailing the nervous system using light microscopy (Halton et al., Reference Halton, Shaw, Maule, Johnston and Fairweather1992; Pax & Bennett, Reference Pax and Bennett1992). Therefore, increased efforts need to be made to study the nervous system of more diplostomid species and other digeneans.

In conclusion, although the studies on the nervous systems of trematodes are difficult to perform under light microscopy, the present study has shown their usefulness in the delineation of species. This proposition is corroborated by the fact that the observed differences in the nervous system of the three metacercariae infecting C. gariepinus were enough to clearly separate them into three species, i.e. T. mashonense, Tylodelphys spp. 1 and 2, as reported using DNA methods (Mwita & Nkwengulila, 2010; Chibwana et al., 2013). In addition, the presence of nerve networks around the pseudosucker regions of both Tylodelphys sp. 1 and Tylodelphys sp. 2, although these species lack pseudosuckers, is an indication that the absence of pseudosuckers could be an advanced evolutionary state in the family Diplostomidae.

Acknowledgements

We are indebted to the Departments of Chemistry and Microbiology and Biotechnology of the University of Dar es Salaam for permission to use some of their chemicals for preparation of cacodylate buffer.

Financial support

This work was supported financially by the Swedish International Development Cooperation Agency (SIDA) – Department for Research Cooperation (SAREC) through the Faculty of Science, University of Dar es Salaam, Tanzania.

Conflict of interest

None.

References

Arafa, S.Z., El-Naggar, M.M., El-Abbassy, S.A., Stewart, M.T. & Halton, D.W. (2007) Neuromusculature of Gyrodactylus rysavyi, a monogenean gill and skin parasite of the catfish Clarias gariepinus . Parasitology International 56, 297–307.CrossRefGoogle ScholarPubMed
Beverley-Burton, M. (1963) New Strigeid Diplostomum (T) mashonense n. sp (Trematoda: Diplostomatidae) from the grey heron, Ardea cinerea L., in Southern Rhodesia with an experimental demonstration of part of the life cycle. Revue Zoologie et de Botanique Afrinaines LXVIII , 291–306.Google Scholar
Chibwana, F.D. & Nkwengulila, G. (2010) Variation in the morphometrics of diplostomid metacercariae (Digenea: Trematoda) infecting the catfish, Clarias Gariepinus in Tanzania. Journal of Helminthology 84, 61–70.CrossRefGoogle ScholarPubMed
Chibwana, F.D., BlascoCosta, I., Georgieva, S., Hosea, K.M., Nkwengulila, G., Scholz, T. & Kostadinova, A. (2013) A first insight into the barcodes for African diplostomids (Digenea: Diplostomidae): brain parasites in Clarias gariepinus (Siluriformes: Clariidae). Infection, Genetics and Evolution 17, 62–70.CrossRefGoogle ScholarPubMed
De Jong-Brink, M. (1995) How schistosomes profit from the stress responses they elicit in their hosts. Advances in Parasitology 35, 177–256.CrossRefGoogle ScholarPubMed
Duvaux-Miret, O., Stefano, G.B., Smith, E.M., Dissous, C. & Capron, A. (1992) Immunosuppression in the definitive and intermediate hosts of the human parasite Schistosoma mansoni by release of immunoactive neuropeptides. Proceedings of the National Academy of Sciences, USA 89, 778–781.CrossRefGoogle ScholarPubMed
Field, J.S. & Irwin, S.W. (1995) Life-cycle description and comparison of Diplostomum spathaceum (Rudolphi, 1819) and D. pseudobaeri (Razmaskin & Andrejak, 1978) from rainbow trout (Oncorhynchus mykiss Walbaum) maintained in identical hosts. Parasitology Research 81, 505–517.CrossRefGoogle Scholar
Grabda-Kazubska, B. & Moczon, T. (1981) Nervous system and chaetotaxy in the cercaria of Haplometra cylindracea (Zeder, 1800) (Digenea, Plagiorchiidae). Zeitschrift für Parasitenkunde 65, 53–61.CrossRefGoogle Scholar
Gustafsson, M.K.S. & Wikgren, M.C. (1981) Release of neurosecretory material by protrusions of bounding membranes extending through the axolemma in Diphyllobothrium dendriticum (Cestoda). Cell Tissue Research 220, 473–479.CrossRefGoogle ScholarPubMed
Gustafsson, M.K.S., Halton, D.W., Kreshchenko, N.D., Movsessian, S.O., Raikova, O.I., Reuter, M. & Terenina, N.B. (2002) Neuropeptides in flatworms. Peptides 23, 2053–2061.CrossRefGoogle ScholarPubMed
Halton, D.W., Shaw, C., Maule, A.G., Johnston, C.F. & Fairweather, I. (1992) Peptidergic messengers: a new perspective of the nervous system of parasitic platyhelminths. Journal of Parasitology 78, 179–193.CrossRefGoogle ScholarPubMed
McKeown, C.A. & Irwin, S.W.B. (1995) The life cycle stages of three Diplostomum species maintained in the laboratory. International Journal for Parasitology 25, 897–906.CrossRefGoogle ScholarPubMed
Musiba, M.J. & Nkwengulila, G. (2006) Occurrence of metacercariae of Diplostomum and Tylodelphys species (Diplostomidae) in Clarias species (Clariidae) from Lake Victoria. Tanzania Journal of Science 32, 89–98.Google Scholar
Mwita, C. & Nkwengulila, G. (2010) Phylogenetic relationships of the metazoan parasites of the clariid fishes of Lake Victoria inferred from partial 18S rDNA sequences. Tanzania Journal of Science 36, 47–58.Google Scholar
Niewiadomska, K. & Moczon, T. (1982) The nervous system of Diplostomum pseudospathaceum Niewiadomska (Digenea, Diplostomatidae). I. Nervous system and chaetotaxy in cercaria. Parasitology Research 68, 295–304.Google Scholar
Niewiadomska, K. & Moczon, T. (1984) The nervous system of Diplostomum pseudospathaceum Niewiadomska, 1984 (Trematoda, Diplostomatidae). II. Structure and development of the nervous system in metacercaria. Parasitology Research 70, 537–548.Google Scholar
Niewiadomska, K. & Moczon, T. (1987) The nervous system of Diplostomum pseudospathaceum Niewiadomska, 1984 (Trematoda, Diplostomatidae). III. Structure of the nervous system in the adult stage. Parasitology Research 73, 46–49.CrossRefGoogle ScholarPubMed
Niewiadomska, K. & Moczon, T. (1990) The nervous system of Diplostomum pseudospathaceum Niewiadomska, 1984 (Trematoda, Diplostomidae). IV. Nervous system and distribution of sensilla in the daughter sporocyst. Parasitology Research 76, 635–637.CrossRefGoogle ScholarPubMed
Nkwengulila, G. (1995) Epidemiology and taxonomy of Diplostomum species (Trematoda: Diplostomatidae) infecting fish of Llyn Tegid, North Wales and Ruvu Basin Tanzania. PhD thesis, University of Liverpool, UK.Google Scholar
Pax, R.A. & Bennett, J.L. (1992) Neurobiology of parasitic flatworms: how much ‘neuro’ in the biology? Journal of Parasitology 78, 194–205.CrossRefGoogle ScholarPubMed
Solis-Soto, J.M. & De Jong-Brink, M. (1995) An immunocytochemistry study comparing the occurrence of neuroactive substances in the nervous system of cercariae and metacercariae of the eye fluke Diplostomum spathaceum. Parasitology Research 81, 553–559.CrossRefGoogle ScholarPubMed
Figure 0

Fig. 1The nervous system of the metacercaria of Tylodelphys sp. 1 from Clarias gariepinus to show (a) the entire body, (b) the forebody and (c) the hindbody; BN, nerves supplying Brandes organ; CG, cerebral ganglion; CN, caudal ventral nerves; DC, dorsal nerve cord; HB, hindbody; LN, nerves supplying pseudosucker region; NB, Brandes organ nerves; NG, nerve connecting cerebral ganglia; NL, nerve loop; NT, nerves supplying excretory pore; ON, nerves supplying oral region; TG, caudal ganglion; PN, peripheral longitudinal nerve cord; TN, transverse nerve; VC, longitudinal ventral nerve cord; VN, nerves supplying ventral sucker; XN, suprapharyngial nerves.

Figure 1

Fig. 2The nervous system of the metacercaria of Tylodelphys sp. 2 from Clarias gariepinus to show (a) the entire body, (b) the forebody and (c) the hindbody. NJ, nerves connecting Brandes organ. See Fig. 1 for the key to the rest of the lettering.

Figure 2

Fig. 3Inverted photographs of entire (a) Tylodelphys sp. 1, (b) Tylodelphys sp. 2 and (c) Tylodelphys mashonense from Clarias gariepinus to show the cephalic and caudal nervous systems. FB, forebody. See Fig. 1 for the key to the rest of the lettering.

Figure 3

Fig. 4The nervous system of the metacercaria of Tylodelphys mashonense from Clarias gariepinus to show (a) the entire body, (b) the forebody and (c) the hindbody. See Fig. 1 for the key to the lettering.