STUDIES IN THE EMBRYOLOGY OF ECHINODERMS. 415 
the latter communication is absent in all the Bipinnarise 
which I have examined, but the dorsal one is usually present. 
In the small form here figured, however, I convinced myself, 
both by observation of the living animal and by sections, that 
no communication existed, between these two cavities at this 
stage. 
It is interesting to notice that this larva gives us two cha- 
racters, not possessed by other Asterid larvae, in which it 
resembles the Plutei of Opliiurids and Echinids : (1) firstly, 
the anterior continuations of the enterocoels are never united 
in the prseoral lobe ; (2) secondly, on the left side there is a 
complete separation of the anterior and posterior enterocoels 
just behind the water-pore. No separation of the anterior and 
posterior enterocoels on the right side was ever observed. It 
should further be noted that in its external form this larva 
gives evidence of being primitive, in that it retains up to the 
time of metamorphosis the simple outline characteristic of all 
young Bipinnarise, without developing the gigantic prseoral 
lobe and elongated arm-like processes which so entirely alter 
the appearance of other Bipinnarise in their later stages. 
In fig. 13 is given a lateral view of the same larva, in which 
it will be seen that the hydrocoel occupies nearly the same 
position as in young Ophiurid and Echinid Plutei namely, 
between the anterior and posterior enterocoels ; but in remain- 
ing open to the anterior enterocoel it approaches the Echinid 
rather than the Ophiurid condition. 
The Bipinnarise with a large prseoral lobe and terminal fin 
(Bipinnaria asterigera), and Brachiolaria, do not differ 
much internally from the form just described, except in the 
already-mentioned communications (1) between the anterior 
and posterior enterocoels of the left side, dorsal to the hydro- 
coel, and (2) between the right and left anterior enterocoels ; 
the latter is clearly secondary. But in Asterina, Ludwig (17) 
describes a very different arrangement ; though, as I shall sub- 
sequently show, my observations do not entirely agree with his. 
According to him, the right and left enterocoels are from the 
first connected with a single large anterior cavity occupying 
