148 



MANUAL OF ZOOLOGY. 



mouth, the two divisions thus produced giving off numerous 

 lateral diver ticula, and terminating posteriorly in blind extre- 

 mities. The nervous, system consists of a ring round the 

 gullet, giving off filaments both forwards and backwards. 



1. 



Fig. 40. Trematoda. 1. Distoma ?iepaficum, the 'Liver-fluke,' showing the 

 branched alimentary canal. 2. Anterior extremity of Distoma lanceolatum. 

 a. Anterior sucker ; b. Posterior sucker ; e. Generative pore ; d. (Eso- 

 phagus ; e. Alimentary canal. (After Owen.) 



In Distoma lanceolatum (fig. 40, 2) the intestine has not the 

 ramose, complex character of that of D. liepaticum. On the 

 other hand, the alimentary canal, after its bifurcation, is con- 

 tinued on each side of the body to the posterior extremity 

 without giving off any branches on the way, and it terminates 

 simply in blind extremities. 



Dvplostomum, in its essential characters, does not differ 

 much from Distoma : but it is found living gregariously in the 

 vitreous humour and lens of the eyes of certain fresh- water 

 fishes, such as the common Perch. 



Other members of the order infest the intestines of birds and 

 Batrachians, the gills of fishes, or the paunch of Ruminants. 



ORDER TURBELLARIA. The members of this order are almost 

 all aquatic, and are all non-parasitic ; thus differing entirelv 

 from the animals which compose the two preceding orders. 

 Their external surface is always and permanently ciliated, 

 and they never possess either suctorial discs, or a circlet of 

 cephalic booklets. A * water-vascular system ' is always 

 present, opening externally by one or many apertures, or ap- 

 pearing to be entirely closed in the adult (Nemertidce) . As 

 in the Trematoda, the alimentary canal is imbedded in the 

 parenchyma of the body, and there is no ' perivisceral cavity.' 

 The intestine is either straight or branched, and a distinct 

 anal aperture may, or may not, be present. 



The Tarbellaria are divided into two sections, termed re- 

 spectijVely the Planarida and the Nemertida. 



SUB-ORDER I. PLANARIDA. The Planarians (fig. 41) are 



