264 ZOOLOGY SECT. 



single posterior aperture ; frequently there are two. In the Tri- 

 cladida there are two longitudinal canals which open on the exterior 

 through special branches by a series of pores. In the Rhabdoccelida 

 there are either two longitudinal main vessels 

 or a single median one ; the communication 

 with the exterior in the former case may be 

 by a pair of ventral apertures, or indirectly 

 through the pharynx ; or there may be a 

 common short passage in which the two 

 trunks unite, opening by a posterior median 

 aperture. When a single main trunk is 

 present it opens at the posterior end of the 

 body. In the Trematodes there are usually 

 two principal longitudinal trunks, which 

 F T^SuSST P"* either unite behind to open at the posterior 

 k. termination of capillary ; en( J o f the body, or (Monogenetica) remain 



n. nucleus ; v. vacuoles ; 



wf. ciliary flame. (After separate and open independently on the 

 dorsal surface, each having, where it opens, 



a contractile excretory sac. In Temnocephala each dorsally opening 

 excretory sac has ramifying through its wall which consists 

 mainly of a single large cell a system of capillary vessels containing 

 ciliary flames. 



In the Cestodes there are usually four longitudinal trunks, which 

 open through a contractile excretory sac at the posterior end of 

 the body. In many cases it has been shown that the main trunks 

 communicate with the exterior at intervals by means of fine canals. 

 The excretory sac is thrown off when the last proglottis becomes 

 separated off and does not in most cases become renewed, though 

 in at least one species of Tape- worm (Tcenia cucumerina) a new 

 vesicle is developed -again and again at the end of the body as a 

 fresh segment is thrown off. The main trunks are connected 

 together by a ring-vessel in the head and in some cases by a 

 transverse branch in each proglottis, and where the latter originate 

 from the main trunks are valves formed by folds of the wall of the 

 vessel. In the posterior region only two of the longitudinal 

 trunks (one on each side) may be retained. 



The sexes are united in all the Platyhelminthes with a few 

 exceptions, and the reproductive organs are sometimes 

 somewhat complicated presenting a remarkable advance on those 

 of the Ccelenterata. The male part of the apparatus consists of 

 testes, with their ducts, the vasa deferentia, often with a contractile 

 terminal enlargement or vesicula seminalis, a cirrus l or a penis, 

 and often prostate or granule glands. The female part com- 

 prises ovary or ovaries, receptaculum, oviduct, uterus, an ootype, 



1 The term cirrus is here restricted to cases in which the terminal part of 

 the male duct, often provided with spines and other chitinous structures, is 

 involuted within a sheath when at rest. 



