Ill 



PLATODES— WATER-VASCULAR SYSTEM 



153 



The whole water-vascular system shows a decided similarity to a 

 greatly developed dermal gland which has sunk deep under the skin, 

 as is often the case in the Platodes (dermal mucus glands, rod glands, 

 accessory glands of the copulatory apparatus). The excretory cells 

 must then be considered as glandular cells, and the canals as glandular 

 efferent ducts. We can actually regard the water-vascular system as 

 a dermal gland, which has undertaken the special function of excretion. 

 In consequence of the strong development of the parenchyma, and 

 especially of the middle layer of the body, and because of the absence 



Fig. 110.— Water -vascular system of various Platodes. A, of Stenostoma; B, of Meso- 

 stoma, after v. Graff ; C, DendroooBlum, after Ijima ; D, Distomum divergens, after Fralpont ; E, 

 PliyllaoantWde, after Plntner. ph, Pharynx ; o, external aperture ; ez, excretory cells (terminal 

 cell) ; eb, contractile vesicle. 



of a body cavity, this gland is obliged to search for the products of 

 excretion all over the body — hence its great ramification. 



Concerning the arrangement of the principal canals and their 

 external apertures (Fig. 110) the following is noteworthy. 



We have no sufl&cient information as to the water-vascular system 

 of the Polyclada. 



In the Triclada (Fig. 110, C) a main canal runs on each side of 

 the body {Gnnda has two such canals on each side, a ventral and a 

 dorsal, bound together by canals), and these open externally on the 

 dorsal side of the body by means of somewhat numerous special 

 branches and excretory pores placed one behind another. These pores 

 are arranged pretty regularly, in Gunda at least, and correspond in num- 

 ber with the lateral intestinal diverticula — the transverse commissures 

 of the nervous system, — in short with the number of those organs 

 which are regularly paired from one end of the body to the other. 



In the Rhabdoccelidce we can distinguish 3 principal types. 



A. There are 2 lateral principal vessels, which have separate 

 external openings on the ventral side, either {a) at the middle or the 

 front part of the body by special branches opening externally direct 

 (Prorhynchidce), or (b) by 2 cross branches which enter the pharyngeal 

 pouch (Fig. 110, B, Mesostomidce, Fortex ?), or (c) direct by two open- 

 ings lying at the posterior end of the body. 



