ECHINOIDEA. 26) 
The blood vascular system is not readily traced, and there is un- 
certainty as to many points. A “dorsal or axial organ” lies beside the 
stone canal, and seems to be connected with a ‘‘genital ring” and 
with a circular vessel around the gullet. There js also a ‘“‘ pseud- 
hzemal” system consisting of « circum-cesophageal sinus with radial 
branches. The fluid cannot be distinguished from that of the body 
cavity ; it contains corpuscles, some of which are pigmented. 
On the area round about the mouth there are ten hollow 
outgrowths, which resemble the skin-gills of starfishes. 
There are also five large vesicles at the top of the lantern 
(Stewart’s organs”) which may function as internal gills. 
As already mentioned, the pigmented cells of the body 
cavity fluid seem able to absorb oxygen. There is no 
doubt that the water vascular system plays a very important 
part in respiration. It probably also aids in excretion. 
The sexes are separate, and indistinguishable externally. 
Five large branched yellow-brown ovaries or rose-white 
testes lie inter-radially under the apex of the shell, and 
open by separate ducts on the five genital plates. In 
spring the apical disc may be seen covered with orange 
ova or milky-white spermatozoa. 
The eggs are fertilised externally by sperms wafted from 
adjacent sea-urchins, and the free-swimming larva is called 
a Pluteus. 
Classification.— 
The class may be divided into three sub-classes or groups of orders. 
Sub-Class I. Regularia Endobranchiata. Mouth and anus at opposite 
poles; the anus surrounded by the apical system of plates if these 
are developed ; no external gills. 
e.g. the somewhat primitive Czdarzs. 
Sub-Class II. Regularia Ectobranchiata. Mouth and anus at opposite 
poles; a double circle of apical plates surrounds the anus ; there 
are external gills, 
e.g. the common genera Zchzzus, Giese Dualtes Arbacia. 
‘The Echinothurine have flexible tests and powerful muscles. 
e.g. Asthenosoma and Phormosoma. 
Sub-Class III. Irregularia. The anus lies outside the apical system 
of plates in the posterior inter-radius. 
e.g. the heart-urchins, Spatangus and Echinocardium, without 
lanterns. In the related Achénoneus there is a lantern in the 
young forms, It is interesting to contrast the large massive 
Clypeaster with the minute EZchinocyanrus pusillus, common 
_in the stomach of cod-fishes. 
