ECHINODERMATA. 
625 
ment within a marsnpium, diffeta therefrom and reminds us of Bipinnaria in 
its want of brachiolar appendages, and in possessing a distinct alimentary 
Canal. The first stages in the emhryogenesis of this form have not yet been 
observed ; but it seems probable that the Echinoderm-disk appears at an 
earlier period than in JEchinaster, and, to a still greater degree, predominates 
over the protozooid. 
If, as Wyville Thomson hints, the development of Asteracanthion glaciaUSf 
observed by Busch, takes place after the manner of Pterastet', then the four 
modes of emhryogenesis to which we have referred are, in all probability, ex- 
emplified by the species of a single genus, Asteracanthion, Which of these 
four modes is the most frequent is a question we cannot decide, since no one 
has yet determined what are those species of starfish which produce the 
several kinds of Bipinnaria. Brachiolaria, however, leads from this form to 
Echinastet' j Echinaster, or rather its protozooid, to Pteraster^ and this, again^ 
to Bipinnaria. 
With special swim- 
ming-appendages. An 
alimentary canal. 
No special swimming 
appendages. 
( Bipinnaria. 
J (A ciliated sac). 
1 Brachiolaria. 
{ (Two ambulacral rudiments.) 
(Echinaster. 
J (No alimentary canal). 
j Pteraster. 
( (No ciliated sac). 
Bipinnaria. 
( Prehensile append- 
(■ ages. 
I No prehensile ap- 
pendages. An aii- 
\ mentary canal. 
Or, starting from Echinaster, we may proceed in two directions, either of 
which conducts to Bipinnaria j and thus, so to speak, we complete the circle 
of “ larval zooids ” among the Asteridea. , Future researches will, doubtless, 
bring to light other forms tending still further to complete the series ; and of 
this kind, it may be conjectured, are those obscure ‘Marvae ” of whose life- 
history but imperfect glimpses have been obtained. It is possible that some 
of the extinct Asteridea presented similar transitional conditions. 
Putting aside, for want of sufficient knowledge, Muller’s Tornaria and two 
curious oceanic Astero-zooids observed by the same naturalist at Nice in 1849, 
the one, his so-called “ vermiform larva,” the other, a ciliated medusiform 
body supposed to be the young of Asteracanthion tennispinosunij we tnay sum 
up the results of past observations on the emhryogenesis of the Asteridea as 
follows : — 
A. The ovum is developed into an oceanic protozooid, furnished "v^ith 
special swimming-appendages. The structures of the protozooid are at first 
wholly independent of those of the deuterozooid to which, by a process inter- 
mediate between gemmation and ordinary metamorphosis, it subsequently 
gives rise. The protozooid possesses an alimentary canal, the stomach and 
intestine of which are appropriated by the deuterozooid. 
1. The structures of the protozooid are cast off by the deuterozooid, and 
may even maintain for a time an independent vitality. The ambulacral ring 
of the deuterozooid commences as a rosette of five caeca, which soon unite 
with one another (Bipinnaria). , 
2. The structures of the protozooid are entirely absorbed by those of the 
1864. [voL. I.] 2 s 
