. Elements of Comparative Anatomy. "\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 bee
. Elements of Comparative Anatomy. "\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 c
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