220 



COMPARATIVE ANATOMY. 





Holothuroida tlie similarly rosette-shaped rudiment forms the oral 

 tentacles, which have therefore an indubitable relation to the ambu- 

 lacra! system (§ 162). The succeeding processes which are of any 

 importance affect the central portion of the rosette, in which the 

 canals of the five leaves have their common orifice. This is con- 

 verted into a circular canal, which continues to form the central 

 portion of the apparatus ; the canals in the leaves of the rosette grow 

 out radially, and extend whilst the number of their lateral branches 

 increases, over the ambulacra, which get larger at the same time. 



The adult stage may be directly derived from these arrangements, 

 formed during the development of the Echinoderm body. A branched 

 vascular system (Fig. Ill) has finally developed from the primitive 

 tube, and has its ends directly connected with the suckers (p) and 

 other such processes. The radial trunks of this system communi- 

 cate with the circular canal (c), and this again is in connection with 



the surrounding medium. We 

 have already mentioned the fact 

 that in Spatangus the water- vas- 

 cular ring around the mouth is 

 connected with an enteric vessel ; 

 and inasmuch as the contents of 

 the two systems of canals are 

 similar, it is very probable that 

 they do not only communicate, 

 but are also to be regarded as 

 parts of one structure. 



The connection with the ex- 

 terior is of a special nature, and 

 is effected in various ways. As the 

 Echinoderm is being differentiated 

 in the larva, that portion of the 

 rudimentary water-vascular system 

 which is taken into the body of 

 the Echinoderm remains connected 

 with the perisome at one spot, 

 where a porous calcareous plate — 

 the madreporic plate (m) — is de- 

 veloped ; this plate communicates 

 with the lumen of that portion of 

 the canal which is connected with 

 it. The duct (m') leading from the 

 madreporic plate to the circular 

 canal, which also is a portion of the 

 primitive water-vascular system, 

 has ordinarily calcareous substances deposited in its walls, and 

 is therefore called the stone -canal; its walls form a complicated 

 cavitary system. Water passes into the stone-canal by the cribriform 

 madreporic plate, and thence to the circular vessel. It has also 

 connections with the ccelom. 



Fig-, ill 

 of the 



Diagrammatic representation 

 ■water-vascular system of a 

 Starfish, c Circular canal, ap Polian 

 vesicles, m Madreporic plate, m 1 Stone- 

 canal, r Radially-arranged principal 

 trunks (Arnbulacral canals), r' Lateral 

 branches, p Suckers, a Their ampullae 

 (part only of the arnbulacral canals and 

 their appendages are figured). 



