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 bursse, 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 Lepadocrinus 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 
