(471). The circular vessel around the mouth, which forms the central receptacle of the vascular system, resembles a sinus analogous to those of the dura mater in man, and is lodged in a groove between the oral circle of vertebrae and the pieces of the skeleton articulated therewith. Connected with the sinus above mentioned, and placed regularly in the interspaces between the rays, are several oval vesicles (fig. 90, k h), filled with a reddish-coloured transparent fluid. These vesicles, which in Asterias au-rantiaca are seventeen in number, communicate by distinct ducts with the central sinus, and are regarded by Delle Chiaje as reservoirs wherein the nutritive fluids accumulate until expelled by the contraction of the vesicles. Besides the arteries above described as arising from the vascular circle around the mouth, according to the author last mentioned, vessels are given off that communicate with the ampullae connected with the ambulacral suckers, apparently for the purpose of supplying to them the fluid which they contain. These vessels are seen to run along the floor of each ray, and to give off lateral branches communicating with every vesicle, as represented in the enlarged sketch (fig. 88, 2,g.) By this arrangement it would seem that the contractile organs (fig. 88, 2, e) appended to the vascular sinus (f) 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.

Asterias auranfiaca opened from above: a, dorsal parietes reflected.

Fig. 90. Asterias auranfiaca opened from above: a, dorsal parietes reflected; b, c, d, floor of the rays, exhibiting the ambulacral vesicles; e, dorsal circular vessel; f, heart; s, circular vessel surrounding the mouth; k k, ampullae Polianae.

(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, Yalentin, Agassiz, Dr. Sharpey, and Muller. 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 compound, under the form of a thickly-corpusculated milky fluid organized in a high degree; and in Sipuneulus 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 muscular 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 musculomembranous appendages, lined within and without by a ciliated epithelium. A few proper blood-vessels reach their bases from the circular vessel; but no trace whatever of a vascular plexus in the structure of these parts can be detected. The inference is that the tentacles are designed for the oxygenization of the chylaqueous fluid. To the genus Holothuria the same observations are strictly applicable; but although attenuated at regular points, with a view to approximate as closely as possible the chylaqueous fluid to the external medium, no open perforation anywhere exists in the tentacular or tegumentary processes.

The surrounding fluid therefore cannot penetrate directly from without into the peritoneal cavity; it is introduced through the mouth and digestive system.

(476). Before quitting this part of our subject, we must briefly mention a singular organ, apparently intimately connected with the circular vessel around the mouth, and called by Tiedemann the sand-canal. This organ is represented in fig. 90, enclosed in the same sheath as the dilated vessel, f, upon the right side of which it is placed; it communicates by one extremity with an isolated calcareous mass, of a rounded figure, called the madreporic plate, seen upon the exterior of the dorsal surface of the Star-fish, while by its opposite extremity it opens into the circular sinus that surrounds the mouth. The tube itself Dr. Sharpey describes* as being about the thickness of a surgeon's probe, and composed of rings of calcareous substance connected by a membrane; so that, viewed externally, it is not unlike the windpipe of a small animal. On cutting it across, it is found to contain two convoluted laminae, of the same nature as its calcareous parietes, which are rolled upon themselves in a longitudinal direction, in the same manner as the inferior turbinated bones of an ox. The convoluted arrangement becomes more complete towards the upper end of the tube, where the internal laminse, as well as the external articulated portion, join the dorsal disk, appearing gradually to become continuous with its substance. The use of this curious organ is quite unknown, although a variety of conjectures have been hazarded upon the subject.