THE STARFISHES 



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in the interradial areas between the ambulacra. From each of these spiracles, a 

 canal passes under the test in a direction away from the mouth. This canal soon 

 branches, and a branch goes to the side of each ambulacrum. Each branch of the 

 canal swells into a pouch with thin walls that are strengthened by a slight deposit 

 of lime; and these walls are thrown into folds so that their surface is increased. 

 There is thus a folded pouch running along the inside of the test under each side of 

 an ambulacrum ; and from this pouch short tubes are given off which open to the 

 exterior through pores at the sides of the ambulacrum, which pores alternate 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 bursae, 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 

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

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

 they are more like the 

 slits previously men- 

 tioned in Lepadocrinus 

 and Porocrinus; while 

 they are always in the 

 same interradial posi- 

 tion, which is not the 

 case with the cystids. 



THE STARFISHES 

 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 upward. There is, however, reason to believe that these free 

 forms are, like the free crinoids, descended from ancestors that were fixed; and in 



BLUE CHINA STARFISH. 

 (Natural size.) 



