16 



ASTEROIDEA. 



elevated likewise to a sufficient height, by cutting across the sand canal and other adhesions to 

 meet the ends of the rays, and thereby form a globe. When the parts are all thus adjusted, 

 it will be seen that they hold the same relation to each other in the Asteriad^e as they do in 

 the test of the EcHiNiDiE. Thus the two flaps from each ray, when united by suture with 

 the flaps from the adjoining rays, represent the wide inter-ambulacral areas, with their 

 zigzag sutures in the centre, and the surface of both being armed with spines increases the 

 analogy. The narrow ambulacra, with their two or four rows of suckers, are undoubtedly 

 the homologues of the ambulacral areas and poriferous zones in the EcJiinidce. The anal 

 area, containing the madreporiform tubercle, genital pores, and the vent, in star-fishes, 

 represents the apical disc formed by the ovarial plates, anal opening, and madreporiform 

 body, in urchins. The five eye-plates at the ends of rays, in star-fishes, will fit into the 

 margin of the circular area, when the extremity of the rays are made to approximate 

 this part, by folding up the flat disc and converting it into a globe, which is the same 

 singular position they occupy in the test of the Fchinidce. The mouth-opening will be 

 obviously the same in position in the under side of the body in both orders. 



If this demonstration is satisfactory, it is clear that we must not seek the homology 

 between the star-fish and sea-urchin by inflating the body of disciform species, and thus 

 making them assume globular forms, as suggested by Miiller, but by placing the homologous 

 parts in the same relation they hold to each other in these two orders of Echinodermata, 

 always recollecting that the test of the EcJdnida forms a hollow globe in which the viscera 

 are enclosed, whilst the skeleton of the Asteriada is a stelliform disc, into each ray of which 

 a portion of the viscera is prolonged. By incising the rays down the centre of their 

 upper surface, and folding down and uniting by suture their sides together, we produce, 

 very clumsily it is true, the same conditions so beautifully provided in the EcMnidce, and 

 reduce to a demonstration the homology of the several parts of which the body of the 

 star-fish is composed. I have selected Vraster rubens for illustration, because the 

 flexibility of the rays enables one to operate upon it easily with the scissors ; but if my 

 reasoning is correct, the observations which apply to this species will hold true with all 

 the others, if they admitted of similar anatomical manipulation. 



The Madreporiform Body. 



The AsTERtADiE, in common with the EcniNiDiE, possess a madreporiform body; 

 which is situated in an excentral position on the upper surface, between two of the rays 

 (fig. 13). From the spongy plate a canal descends towards the mouth. In Bchinida, 

 the madreporiform body always occupies the right antero-lateral ovarial plate ; and as I 

 have shown that the single ambulacral segment represents the anterior part of the body in 

 the sea-urchins, for this reason it is inferred, that the single ray to the left of the madre- 

 poriform body in the species possessing a single plate forms the homologous part of 

 the animal in the star-fishes. 



