130 CLASS iv. 



Notwithstanding much careful investigation, there still exists 

 great obscurity about the circulation of the blood in Ecliinoderms. 

 TIEDEMANN and DELLE CHIAJE give very conflicting descriptions 

 of the vascular system the difference being founded in the inter- 

 pretation of the dermal vessels, which are connected with the 

 organs of motion. The first of these authors considers the motion 

 of fluid observed in these vessels to be altogether distinct from the 

 circulation, whilst, according to the other, they are a part of the 

 system of blood-vessels. In Asterias TIEDEMANN found on the 

 inner surface of the skin of the back, a vascular ring, which he 

 considers to be venous. The vessels which run upon the surface 

 of the visceral appendages of the rays open into this ring. From 

 it a canal arises, which performs the office of a heart, lying near 

 the so-called lime-canal which is found there. The canal runs 

 into a vascular circle surrounding the mouth, which TIEDEMANN 

 holds to be arterial, and from which branches proceed to the 

 intestines. Besides these two vascular rings (one on the dorsal 

 and one on the abdominal surface), there is a third ring of an 

 orange-yellow colour found on the inferior surface beneath the skin. 

 TIEDEMANN was not able to discover any communication between 

 this ring and the rest of the vascular system. In Echinus vascular 

 rings occur, in like manner, round the mouth and the anus, on each 

 surface two, of which one is to be considered arterial, the other 

 venous. The heart is oblong, divided into many cells, and lying 

 on the oesophagus 1 . In Holothuria there is a circulating system 

 without a heart, or rather the heart has the form of a contractile 

 vessel, that runs at the outside on the surface of the intestine. 

 At the anterior extremity of the intestinal canal this vessel forms a 

 vascular circle, whence very fine branches arise ; when near the 

 anus it has become small, having given off a multitude of fine 

 branches, which run on the surface of the intestine. There is a 

 transverse vessel which connects the longitudinal trunk on the first 

 loop of intestine with that on the second. Many intestinal veins, 

 which seem at the same time to perform the part of absorbents or 



1 Comp. the descriptions and figures of VALENTIN, Anatomic du genre Echinus, 

 pp. 8996. Tab. vii. fig. 119, 125, 127. Tab. vm. fig. 144152, &c. There is a 

 figure also of the heart and part of the blood-vessels in Spatangus in CUVIER'S Regne 

 Anim. $d. illustree, Zoophytes, PL n bis. 



