"\YATEE-^^l:ssELS or echinoderma. 



219 



Water-Vessels. 



§ 177. 



lu describing the ambulacra (§ 160), mention was made of a 

 "water-vascular system," wbichtook in water from tbe exterior, 

 and carried it to tbe ambulacral organs, wbicb it put into the condition 

 of erection. Otter organs in addition to the structures which take 

 part in locomotion are filled by this system of canals ; and these we 

 have already spoken of as modifications of the ambulacral feet. The 

 probability of this system of canals being a portion of the blood- 

 vascular system has been already pointed out. Communications 

 have been noticed at several points; and in some cases openings 

 into the coelom also have been distinctly observed. It is, however, 

 not yet certain how far these vessels have been formed from other 

 organs. In any case we must still regard the water-vascular system 

 as being independent, especially as its development shows that it 

 is so, and as an important division of 

 the system (stone-canal, etc.) arises as 

 a structure, which is primitively 

 quite independent of the circula- 

 tory system. 



In the laiwse of the Echinoderma the 

 water-vascular system is formed by a 

 differentiation from the earliest rudiment 

 of the enteron ; as it gets nipped off, it 

 forms a transparent tube, ciliated in- 

 ternally, and connected with the integu- 

 ment on the back of the larva, where it 

 soon opens by a pore. When in this 

 condition the organ has a close resem- 

 blance to the excretory organs in the 

 larvse of many Vermes (SipuncuHdse), 

 so that from this point of view it does 

 not seem improbable that the water- 

 vascular system has been differentiated 

 from a primitive excretory apparatus. 



This tube, with the other rudiments 

 of the Echinoderm (Fig. 110, A), becomes 

 gradually surrounded by the perisome; 

 it then changes its form by becoming 

 metamorphosed into a five-leaved rosette 

 (i). The portion which still continues 

 to open to the exterior by the dorsal pore gradually changes its 

 position, and gets to lie on the ventral surface of the Echinoderm ; 

 each leaf of the rosette is now developed into an elongated canal 

 with lateral diverticula; it is Kke a pinnate leaf, and forms the 

 rudiments of the ambulacral portion of the water-vessels. In the 



Fig. 110. Larva of an Asterid 

 (Bipinnaria) with budding 

 Echinoderm. eel d' g g' Pro- 

 cesses of the body, h Mouth, 

 o Anus of the larva. A Body 

 of the embryonic Echinoderm. 

 h Ciliated tube, i Ambulacral 

 rosette (Eudiments of the water- 

 vessels) (after J. Miiller). 



