644 THE ANIMAL KINGDOM. 



two or three pairs backwards. Of the latter, one pair is dorsal and 

 submedian, and extends in length to a variable degree, sometimes 

 anastomosing posteriorly (D. isostomum}. A second pair is perhaps best 

 described as ventral or ventro-lateral, and the third, present e. g. in 

 Tristomum Molae, D. isostomum, D. palliatum, may be termed lateral. The 

 two ventro-lateral nerves may unite posteriorly. In Tristomum Molae 

 D. isostomum, D. clavatum circular commissures connect these longitudinal 

 nerves from place to place. In some species of Distomum the ventral 

 nerves possess a strong transverse commissure in front, and another behind 

 the ventral sucker to which fibres pass from them. It is probable that 

 the Trematoda in general will be found to conform to the type of nervous 

 system described. Ganglion cells occur in the nerves as well as in the 

 ganglia. 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 Polystomtim 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. 



The mouth is usually situated anteriorly, either terminally, or on the 

 ventral aspect. In Caster ostomum 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 oeso- 

 phagus \ the digestive tube up to this point being lined by a cuticle, 

 a continuation inwards of the cuticle of the body. In Amphistomum and 

 G aster odiscus, 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 Gaste- 

 rostomum, 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 Bilharzia, 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, Tristomtim, Pseudocotyle. 



1 It 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 Reditu 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). 



