3°4 



E CHINODERMS. 



with the arms and are not in any way connected with them. These hydrospires 

 are clearly quite different structures to either of the two kinds of structures that 

 go by the same name in the Cystidea. They have been compared with certain 

 structures in the Ophiuroidea. In the latter animals are oval pouches, which lie at 

 the sides of the arms where they spring from the body or disc, and which open to 

 the exterior by slits. Their walls are strengthened by calcareous rods, and into 

 them the ovaries open, so that developing young are often found in them, as in the 

 marsupium of a kangaroo. They are known as genital bursas, but their folded 

 inner walls probably serve to bring the outer aerated water closer to the internal 

 organs of the body, that is to say, their function is in part respiratory. We may, 

 therefore, fairly suppose that the hydrospires of Blastoids served primarily for 

 respiration, possibly in place of tube-feet, and secondly for the maturation and trans- 

 mission to the exterior of the generative products. All blastoids have not 

 hydrospires of precisely the same structure as those above described, since in some 

 they are more like the slits previously mentioned in Lepadocriniis and Porocrinus; 

 while they are always in the same interradial position, which is not the case with 

 the cystids. 



The Star-Fishes, — Class Asteroidea. 



With the Asteroidea we come to echinoderms that differ from those hitherto 

 described, and resemble those to be dealt with, in that none of them are fixed, but all 

 are free-moving, and in the fact that the mouth is not directed upwards. There is, 



however, reason to be- 

 lieve that these free 

 forms are, like the free 

 crinoids, descended from 

 ancestors that were 

 fixed ; and in the young 

 Asterina, at all events, 

 there is a prolongation 

 of the forepart of the 

 body, which not only 

 corresponds in position 

 to the prolongation that 

 becomes the stem in 

 crinoids, but actually 

 serves for a short time 

 as an organ of attach- 

 ment. But whereas in 

 the crinoid the mouth 

 moves upwards to the 

 surface opposed to this 

 organ of attachment, and there becomes surrounded by arms, which similarly face 

 upwards, in the asteroid the mouth and its surrounding arms are bent downwards 

 so as to face the sea-floor, and the animal, instead of collecting its food from the 

 water above, extracts it from the mud below. Correlated with this mode of life, the 



