156 ECHINODERMATA. 



vertical tube of communication (/), which, from its muscular ap- 

 pearance and great irritability, Tiedemann regards as being equiva- 

 lent in function to a heart. The circle around the mouth (s) 

 would seem to be arterial in its character ; and from it branches 

 are derived which supply the various viscera of the body. 



But besides the vessels above described, apparently so disposed 

 as to collect and distribute the nutrient fluids, there is another 

 set of canals appropriated to the supply of the numerous vesicles 

 connected with the locomotive suckers ( 190) ; these Tiedemann 

 regards as being totally unconnected with the vascular system 

 properly so called, and considers the fluid contained in them as quite 

 of a different nature. Delle Chiaje, on the contrary, asserts that 

 the two sets of vessels are derived from each other, and describes a 

 peculiar apparatus connected with them as performing an important 

 part in effecting the protrusion of the suckers. 



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 articu- 

 lated therewith. Connected with the sinus above mentioned, and 

 placed regularly in the interspaces between the rays, are several 

 oval vesicles (Jig. 67, &, &), filled with a reddish-coloured transpa- 

 rent fluid. These vesicles, which in dsterias aranciaca are seven- 

 teen in number, communicate by distinct ducts with the central 

 sinus, and are regarded by Delle Chiaje as reservoirs in which 

 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 which communicate with the am- 

 pulls6 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 (Jig. 62, 2 g) . By this arrangement it would 

 seem that the contractile organs (Jig. 65, % e.) appended to the 

 vascular sinus f, are in reality antagonists to the tubular structure 

 of the feet, and serve as receptacles for fluid, which, by their con- 

 traction, they can force into the whole system of locomotive suckers 

 whenever the feet are brought into action. 



The above view of the arrangement of the vascular system of 

 Asterias is, however, by no means universally admitted to be cor- 



