ECHIHODERMATA. 



43 



this vessel or its branches and the rest of the 

 vascular system, and he professes himself at a 

 loss to conjecture what may be its function. 



According to Delle Chiaje the circular ves- 

 sel (i, i,fgs. 12 and 22,) into which the canals of 

 the feet open receives also the veins from the up- 

 per surface of the coeca and stomach. The same 

 vessel, which he names the venous sinus, gives 

 out 1 . twenty short dental arteries ; 2. the mesa- 

 raics to the under surface of the cceca; 3. five 

 vertebral arteries which open into the vesicles 

 of the feet; 4. the radial to the under part of 

 each ray ; 5. the dorsal arteries to the upper 

 part of the ray, which extend their ramifications 

 to the external surface of the body. 



b. Echinus. A circular vessel, supposed 

 to be of a venous nature, surrounds the anal 

 extremity of the intestine (fg. 19, at c), being 

 situated on the internal surface of the shell. 

 A vertical vessel (e, cut short) descends from 

 it towards the lantern and opens into a short 

 oval canal (A) with muscular parietes, which 

 exhibits during life slow but distinct contrac- 

 tions and dilatations, and which is therefore 

 considered as a heart. The heart is situated 

 near the commencement of the intestine ; 

 a vessel (i, i, i, ') issues from it which first 

 sends branches to the oesophagus and the 

 muscles and membranes of the lantern, and 

 then runs along the whole intestine on its inner 

 border, first increasing somewhat in diameter, 

 afterwards gradually diminishing as it ap- 

 proaches the anus, where it terminates. This 

 vessel gives off at all points of its course small 

 branches to the intestine; it contains a dark 

 yellow fluid coagulable by alcohol, and its 

 parietes contract on mechanical irritation ; 

 Tiedemann conceives it to be an artery. Ano- 

 ther vessel (/x, k, k, k) equal in length to the 

 last described, but not directly connected with 

 the heart, runs along the intestine on its outer 

 or mesenteric border; it also is widest in the 

 middle of its course, from whence it may be 

 traced in one direction as far as the lantern, 

 and in the other to the vicinity of the anus. 

 Along its whole course this vessel receives 

 small branches from the intestine, and gives 

 off branches from its other side, which pass 

 along the mesentery to the internal surface of 

 the shell, and are ramified on the lining or 

 peritoneal membrane. Tiedemann regards this 

 vessel as a vein ; but as it does not directly 

 communicate with either the heart or the cir- 

 cular vessel, he conceives that the fluid which 

 it circulates is conveyed into it by one set of 

 branches, and out of it by the other, the in- 

 testinal being its entering and the mesenteric 

 or peritoneal its issuing branches. Lastly, the 

 circular vessel placed round the termination of 

 the intestine receives several vessels which 

 come from the peritoneal lining of the shell, 

 and whose commencing branches are probably 

 continuous with the terminations of the peri- 

 toneal branches from the longitudinal vein. 

 Tiedemann conceives the circulation to take 

 place in the following manner. The blood 

 passes from the circular vessel into the heart ; 

 it is then propelled along the artery and its 

 branches ; from these it passes into the veins 



of the intestine, which also absorb the chyle, 

 and the mixed fluid is conveyed into the great 

 longitudinal vein; it next passes into the 

 branches of this vessel, which are distributed 

 to the lining membrane of the shell, and is at 

 last conveyed back by another set of vessels 

 into the circular vein, from which we have 

 supposed it to set out. That this is the course 

 of the circulation is inferred from the anatomy 

 of the circulating organs. On similar grounds 

 Tiedemann with great probability supposes that 

 the blood undergoes its respiratory change, 

 at least chiefly, in its passage through the 

 vessels of the peritoneal membrane, being 

 there most effectually exposed to the influence 

 of the water; he accordingly compares the 

 branches of the great vein which ramify on 

 that membrane to pulmonary or branchial 

 arteries, and the vessels which return the blood 

 to the circular vein, together with that vein 

 itself, to pulmonary veins. He found that the 

 fluid contained in the longitudinal vein was 

 of a yellowish white colour, from which cir- 

 cumstance, as well as from the fact that he 

 could discover no special chyliferous vessels, 

 he inferred that the chyle was absorbed by its 

 intestinal branches. This vein did not con- 

 tract on the application of stimuli. 



Delle Chiaje's description of the vessels 

 of the Echinus is in substance as follows. An 

 annular vessel surrounds the oesophagus ; it 

 receives the termination of the intestinal vein, 

 and gives out the intestinal artery, which like 

 the vein runs along the intestine, and also five 

 cesophageal arteries, which before ramifying on 

 the mouth communicate (by means of a branch 

 passing between the muscles of the teeth) with 

 the dorsal arteries. These last are the canals 

 of the feet; they run along the ambulacra to 

 the anus, where, according to Delle Chiaje, 

 they form a ring, and in their course send 

 lateral branches into the feet. 



c. Holothuria. A vessel (jig- 20, i, i, i, ',), 

 which Tiedemann conceives to be the great 

 artery, runs along the free border of the intes- 

 tine. It is widest in the middle, and gradually 

 disappears posteriorly in the neighbourhood of 

 the cloaca, while anteriorly it forms an annular 

 vessel (at e) round the stomach, out of which 

 branches proceed to the stomach, the ovaries 

 and the sac connected with the canals of the 

 feet and tentacula formerly described. A short 

 but wide anastomosing branch (cut at k, /c,) 

 passes from the artery about the middle of the 

 first portion of the intestine, to join it again at 

 the middle of the second portion (w), that is, 

 nearly about the middle of the arterial trunk 

 itself. Slow contractions, followed by dilata- 

 tions, were observed by Tiedemann in this 

 vessel; they commenced at the middle or 

 widest part, and proceeded in opposite direc- 

 tions to its two extremities, carrying on the 

 light brown-coloured blood contained within it 

 in a corresponding manner. The main artery, 

 which seems thus also to serve the purpose of 

 a heart, sends in its course numerous branches 

 to the intestine, from these the blood is received 

 by the commencing veins, which, uniting to- 

 gether at the opposite or attached border of the 



