Large cells furnished with processes are scattered among the bundles of muscular fibres. In Tristomum nerve-fibrils were traced by Lang into continuity with these cells, but Looss was unable to demonstrate a similar connection in D. palliatum, and regards them as connective tissue cells. Eyes are present in Tristomum and Polystomum among monogenetic Trematoda, four in number, situated dorsally and anteriorly upon or within the ganglia. In T. Molae they consist of a mass of pigment, a refractile body, and a nerve terminating (?) in a ganglion cell. No other organs of special sense occur.
1Kerbert found in Distomum Westermnani (I) a delicate cuticle, (2) a layer of cells, (3) a basement membrane to which the spines were attached. (1) and (2) were sometimes missing. Hence he concludes that the Trematode cuticle = a basement membrane, A. N. A. xix. 1881, p. 531. But Ziegler is convinced (Z. W. Z. xxxix. 1883, pp. 542-7) that the cuticle is a metamorphosed layer of cells. Nuclei are to be found in it in Bucephalus, according to him. So too in Sporocysts and Cercariae, according to Biehringer (Arb. Zool. Zoot. Inst. Wurzburg, vii. 1885, p. 4). Schwarze agrees in the same view (Z. W. Z. xliii. 1886), and, according to him, the cuticle of the oesophagus, excretory vesicles, and main excretory canals are similarly derived. The walls of the oviduct in some Distomidae are cellular.
The mouth is usually situated anteriorly, either terminally, or on the ventral aspect. In Gasterostomum it is median and ventral. It is surrounded by a sucker in most Distomeae, but in the Tri- and Poly-stomeae it leads either directly into the pharynx, or into a vestibule, which may be armed with a sucker on either side, e. g. Axine, Microcotyle. A pharynx with strong muscular walls is rarely absent, as e. g. in Bilharzia, Distomum reticulatum, and it is followed by a longer or shorter oesophagus1, the digestive tube up to this point being lined by a cuticle, a continuation inwards of the cuticle of the body. In Amphistomum and Gasterodiscus, the pharynx is not specialised, and the oesophagus has strong muscular walls, and is furnished with a right and left caecal pouch. The digestive tract itself is either a simple sac, as in Aspidogaster and Gasterostomum, or a forked tube, one limb of the fork running backwards close to each side of the body. The limbs of the tube may end blindly, or unite posteriorly as in Monostomum, the female Bilkarzia, some Tri- and Polystomeae. The tubes themselves may be simple, or furnished with caeca, small as in D. palliatum, D. Megnini; short and wide as in many Polystomeae, long and branched as in Fasciola, Tristomum, Pseudocotyle.
1It is perhaps open to doubt whether the muscular structure at the commencement of the digestive tract in Monostomum and Gasterostomum is a pharynx or an oral sucker. Schwarze considers the pharynx, so-called, of Rediae as a sucker. The pharynx is sometimes protrusible, e g. in Udonella, and the Redia of F. hepatica uses it to devour the liver of its host (Thomas).
They are connected in Polystomum integerrimum by three transverse anastomoses, also beset with caeca. The tract is lined by cells the shape, size, and other characters of which appear to vary, not only in different Trematoda, but according to the state of the digestive cavity whether full or empty. Two muscular coats, an internal circular, and an external longitudinal, more or less well-developed, surround the digestive part of the tract. There is no anus. Unicellular salivary glands open into the pharynx in Polystomum integerrimurn.
The excretory system consists in the digenetic Trematoda of a terminal or contractile vesicle, a system of canals, with terminal canalicules and ciliated funnels or flame-cells (see p. 581). The last-named have been recognised in a number of Trematoda, and they are without doubt universally present. The canalicules leading from the funnels have been seen to open in bundles near to one another into the system of canals. Close to their apertures they are said to anastomose. The canals are variously disposed, and their several sections, large or small, have a proper calibre throughout. Two at least, and sometimes more main canals are as a rule connected with the terminal vesicle. When there are two, they correspond one to either side of the body. The branches they give off either remain independent or anastomose in various ways. A net-work of vessels may be thus established as in D. reticulatum or F. hepatica. The terminal or contractile vesicle opens at or near the posterior extremity of the body, sometimes somewhat dorsally. It may be cylindrical or T-shaped, and a short narrow tube usually connects it to its pore.
Small vessels occasionally originate from it, and in F. hepatica the vesicle itself appears to be drawn out into a long median dorsal canal giving origin to numerous branching vessels which form a reticulum over the posterior two-thirds of the body. The walls of the canalicules, canals and pulsatile vesicle are formed by a delicate structureless membrane or cuticle. It is possible that this membrane may represent a layer of metamorphosed cells1. The contents of the system is a liquid, sometimes coloured, and containing a variable number of large or small granules in suspension. The latter appear to be calcareous in nature. In Diplostomum volvens and rhachiaeum, the main canals and their branches give origin to short and either simple or dichoto-mously branched tubes, which terminate in small bulbs. Every bulb contains a calcareous body very similar to the structure so-named in Cestoda.
1 Cells forming the walls of the pulsatile vesicle have been observed by Schwarze and Fraipont; the latter also observed them in the main canals of D. squamula; so too Looss in D. reticulatum. Cilia were seen by Cunningham in the canals of Stichocotyle, and have been described in other instances, e. g. by Zeller in Polystomum integerrimtim; by Lorenz in Axine and Microcotyle; by Looss in D. reticulatum, etc. See authorities cited by Fraipont, Arch, de Biologie, i. 1880, p. 417 18.