Elements of comparative anatomy (1878) Elements of comparative anatomy elementsofcompar00gege Year: 1878 WATER-VESSELS OF ECHLNODERMA. 221 The portion corresponding to the stone-canal is not always con- nected with the perisome. In the Holothuroi'da the connection is broken close to the dorsal pore of the larva; the latter disappears, and the stone-canal hangs freely in the body-cavity, whence it takes up water by a very complicated and porous terminal apparatus. There are further complications of the water-vascular system, duo to the formation of contractile diverticula of the water-canals p
Elements of comparative anatomy (1878) Elements of comparative anatomy elementsofcompar00gege Year: 1878 WATER-VESSELS OF ECHLNODERMA. 221 The portion corresponding to the stone-canal is not always con- nected with the perisome. In the Holothuroi'da the connection is broken close to the dorsal pore of the larva; the latter disappears, and the stone-canal hangs freely in the body-cavity, whence it takes up water by a very complicated and porous terminal apparatus. There are further complications of the water-vascular system, duo to the formation of contractile diverticula of the water-canals projecting iuto the body-cavity; these must be mentioned in addi- tion to the arrangement, just sketched. These diverticula vary greatly in character; on the circular canal they form large pear- shaped vesicles (Polian vesicles) (ap); where the ambulacral canals pass into the sucking feet they form small ampul las (a), which always project into the body-cavity, and which may be regarded as enlargements or diverticula of the branches of the ambulacral canals. They are cavernous in structure. Both these kinds of organs serve as receptacles for the fluid passing into the canals, and owe their structure to their adaptation to the function of this vascular system; that is to say, when the suckers are drawn hi, their ampullae are always filled, and when the suckers are pro- truded the contents of the ampullae swell them out. What the ampullae are for the separate suckers, the Polian vesicles of the circular canal are for the whole system of canals; that is, they allow of a much more rapid action of the ambulacral structures, whether these are pushed out or drawn in, than would be possible if the quantity of fluid needed for the erection of each separate sucker must be first taken in either by the stone-canal or the madreporic plate. This activity of the ampullae of the suckers and of the Polian vesicles of the circular canal is due to the contractility of their walls, in which a muscu
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