PHYLUM PLATYHELMTNTHES 



269 



The only vascular system present in the Platyhelminthes is the 

 system of 'water-vessels (protonephridia) which are commonly regarded 

 as performing an excretory function. The arrangement of these, 

 the mode of ending internally of the finest branches, and the way in 

 which the system communicates with the exterior, vary greatly in 

 the different groups. A series of main longitudinal trunks give 

 off branches which subdivide to form a system of minute inter- 

 lacing branches or capillaries. In little spaces at the ends of the 

 capillaries are a number of highly characteristic structures the 

 ciliary flames. Each ciliary flame consists of a bundle of vibratile 

 cilia ; typically each is situated in the interior of a cell the 

 flame- cell (Fig. 214) terminating one of the capillary branches. 

 But there are some cases in which there are 

 several flames in each flame-cell. The finer 

 branches, and in some cases the larger trunks 

 also, are intra-cellular, and are to be looked 

 upon as perforations in linear rows of 

 elongated cells. In the Cestoda, at least 

 the larger trunks are inter-cellular, being 

 lined by an epithelium of 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 ; some- 

 times, 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 usually 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 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 



;j. 214. Flame-cell of a 

 Turbellarian. /. processes ; 

 k. termination of capillary ; 

 n. nucleus ; v. vacuoles ; 

 wf. ciliary flame. (After 

 Lang.) 



