180 ECHINODERMATA. 



ing to the author last mentioned, vessels are given off that communi- 

 cate with the ampullae connected with the ambulacral suckers, appa- 

 rently for the purpose of supplying to them the fluid which they con- 

 tain. These vessels are seen to run along the floor of each ray, and to 

 give off lateral branches communicating with every vesicle, as repre- 

 sented in the enlarged sketch (fig. 88, 2,#). By this arrangement it 

 would seem that the contractile organs (fig. 88, 2, e) appended to the 

 vascular sinus (/) are in reality antagonistic to the tubular structure of 

 the feet, and serve as receptacles for fluid, which, by their contraction, 

 they can force into the whole system of locomotive suckers whenever the 

 feet are brought into action. 



(472.) The above view of the arrangement of the vascular system of 

 Asterias is, however, by no means universally admitted to be correct. 

 Professor Sharpey agrees with Tiedemann in the opinion that the vessels 

 of the feet form a system perfectly distinct from that of the blood-vessels, 

 and even supposes that the fluid by which the ambulacral tubes become 

 distended is neither more nor less than pure sea-water. 



(473.) In the Echinodermata therefore there are, 1st. The cavity of 

 the body (i. e., the spacious interval which separates the digestive from 

 the tegumentary system), filled with a fluid designated chylaqueous. 

 2nd. The protrusile suctorial feet, occupied by another class of fluid 

 (this system constitutes the water- vascular system of Tiedemann and 

 Miiller). 3rd. The blood-vascular system of Tiedemann, Delle Chiaje, 

 Valentin, Agassiz, Dr. Sharpey, and Miiller. These three systems are 

 generally regarded as distinct and independent. 



(474.) The mass of fluid occupying the visceral cavity of the Echino- 

 derms (bounded on one side by the digestive system, on the other by the 

 integuments) has been generally described as consisting purely of sea- 

 water admitted directly from without, through the skin, for the exclusive 

 purpose of aerating the blood proper, said to circulate in a capillary 

 system of vessels wrought in the solid parietes circumscribing the 

 cavity. In the Asteridae, Echinidae, Ophiuridae, and Ophiocomidae, it 

 cannot be denied that the cavity itself is the anatomical homologue of a 

 real perigastric cavity ; while in the Holothuridan and Sipunculidan 

 genera it presents itself as a chamber filled with a chylaqueous com- 

 pound, under the form of a thickly-corpusculated milky fluid organized 

 in a high degree ; and in Sipunculus it would seem that the cephalic 

 appendages, as well as the whole tegumentary system, are organized with 

 especial reference to the aeration of this fluid. 



(475.) The skin is fenestrated ; that is, at regular intervals the mus- 

 cular layer disappears, and an interval results, of elliptical figure, covered 

 by only a single layer of epidermis. It is a simple musculo-membranous 

 partition intervening between the chylaqueous fluid within and the 

 surrounding element without ; and through this veil the two divided 

 fluids interchange their gases. The tentacles are merely hollow musculo- 



