234 ECHINODERMA. 



Vascular System. We have not yet reached certainty in regard to 

 this system. German authorities, e.g.* Ludwig, describe (i) a radial 

 blood vessel above the nerve in each arm ; (2) a circumoral vessel 

 around the mouth ; (3) a heart lying beside the stone canal and leading 

 into (4) an aboral ring which gives off vessels to the genital organs. 



But others say that the so-called " heart " is a solid glandular organ, 

 that the aboral ring is merely the connecting strand or rhachis of the 

 genital organs, and that the radial and circumoral vessels described are 

 really thickened septa within the true vessels. 



French authorities describe (a) a radial perihremal space or blood 

 vessel divided by a median mesentery, and (b] the union of these in a 

 circumoral ring. But the latter encloses (c) another annular vessel with 

 which a sinus (d) surrounding the stone canal communicates. Finally, 

 an aboral pentagon (e) gives off five pairs of genital blood vessels. 



Respiratory System. 



From the dorsal surface and sides of a starfish in a pool, 

 numerous transparent processes may be seen hanging out 

 into the water. They are the simplest possible respiratory 

 structures, contractile outgrowths of the skin, with cavities 

 continuous with the ccelome, and are called " skin gills." 

 It is likely that pigmented cells of the body cavity fluid act 

 like rudimentary red blood corpuscles ; the water vascular 

 system may help in aeration ; and the whole body is of 

 course continually washed with water. 



Excretory System. 



The u skin gills" are said to have an excretory function ; for 

 phagocytes, bearing waste, seem to traverse their walls. It 

 may also be that excretion is somehow concerned in forming 

 the carbonate of lime skeleton, but facts are wanting. 



Reproductive System. 



The sexes are separate, and they are like one another, 

 both externally and internally. The organs develop periodi- 

 cally, and lie in pairs in each arm. Each is branched like 

 an elongated bunch of grapes, and is surrounded by a blood 

 sinus. Each has a separate duct, which opens on a porous 

 plate, between the bases of the arms on the dorsal surface. 

 In Asterina gibbosa, however, the eggs are extruded ven- 

 trally. The eggs are fertilised in the water, and the free 

 swimming larva, which will be described along with those of 

 of the other classes, is known as a Bipinnaria or as a 

 Brachiolaria. 



