ex* 



fe 



254 ZOOLOGY SECT 



small cells. This system of water-vessels opens on the exterior 

 in a variety of different ways : sometimes it opens by a number 

 of minute pores ; sometimes, as in the Liver-fluke, there is a single 

 posterior aperture; frequently there are two. In the Tricladida 

 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 two principal longitudinal 

 trunks, which either unite behind to open at the posterior end of 

 the body,or(Monogenetica) remain separate and open independently 

 on the dorsal surface, each having, where it opens, a contractile 



'cretory sac. 



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 (Tccnia 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 by a transirr*: Iranch 

 in each proglottis, and where the latter originate from the main 

 trunks are valves formed by folds of the wall of the vessel. 



The sexes are united in all the Platyhelminthes, with only 

 one or two exceptions, and the reproductive organs are 

 sometimes somewhat complicated presenting a remarkable ad- 

 vance on those of the Ccelenterata. The male part of the 

 apparatus consists of testes, with their ducts, the vasa dcfcrcntia, often 

 with a contractile terminal enlargement or vesicula scminalis, a 

 cirrus or penis, and often prostate or granule glands. The female 

 part comprises ovary or ovaries, receptaculum seminis, oviduct, uterus 

 or ootype, often a bursa copulatmx, shell glands, vitelline or yolk 

 glands, and cement glands. In most, though not in all, there is an 

 ovary or ovaries in which the ova are formed, and a set of vitelline 

 glands or yolk glands producing material which surrounds each of 

 the mature impregnated ova before it becomes enclosed in its 

 shell. The shell glands secrete the chitinoid substance of the egg 

 shells. The cement glands secrete a viscid material for causing 

 the eggs to adhere together, enclosing them in a cocoon or fasten- 

 ing them to some foreign body. The oviduct is the passage by 

 which the ova reach the exterior from the ovary, but an enlarged 



