TREMATODA. 645 



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 integerrimum. 



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 cells 1 . 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 Z>. reticulatum. 

 Cilia were seen by Cunningham in the canals of Stichocotyle, and have been described in other 

 instances, e. g. by Zeller in Polystomum integerrimum ; by Lorenz in Axine and Microcotyle ; by 

 Looss in- D. reticulatum, &c. See authorities cited by Fraipont Arch, de Biologic, i. 1880, p, 

 417 18. 



