624 
ZOOLOGICAL LITERATURE. 
to the larval feet of Echinaster sanguinolentus and Aster acanthion 
mulleri. A like view of their nature had previously, it is true, 
been expressed by Muller himself*. 
These investigations of Sars and A. Agassiz are of importance, since they 
not only constitute positive additions to our knowledge, but enable us also 
to confirm the conjectures of previous observers as to the mutual relations of 
the different modes of development which occur among the Asferidea. No 
previous account had been given of the earliest stages in the life-history of 
Bipinnaria or Brachiolaria, nor had any one of these ‘‘ larvse ” been referred 
to its particular species of starfish. 
In this order of Echinodermata^ the protozoid (“larva,” “larval zoid,” 
“ pseudembrvo ”), or immediate product into which the fecundated ovum 
evolves itself, appears under two strongly contrasted forms. The first is well 
known as Bipinnaria. The second is exhi bited by the young of Echinaster^ 
and of some spepies of Asteracanthion. Bipinnaria, to an extraordinary degree, 
seems independent of the Echinoderm-disk which it produces, while Echi- 
nastet' is so related to its young that the very existence of a distinct “larval 
zoi'd ” has been questioned, the development of this starfish being likened 
to the metamorphosis which prevails among the ordinary marine Inverte- 
hrata f. 
Now these dissimilar forms are plainly connected by Brachiolaria, which, 
albeit its close resemblance, both in structure and habit, to Bipinnaria, agrees 
with the young of Echinaster (1) in its prehensile appendages, and (2) in the 
circumstance that it does not detach itself as a separate zoid from the young 
Echinoderm, but is wholly absorbed or disappears. The intermediate nature 
of Brachiolaria is rendered still more clear when we consider the lime of 
appearance and relative bulk of the Echinoderm-disk which it evolves. Both 
in Brachiolaria and the young Echinaster, a cavity homologous to the ciliated 
sac of Bipinnaria exists j and this, though a structure of the protozoid, is 
nevertheless brought into direct connexion with the rudimentary ambulacral* 
system. And in all three forms of “ larvae,” the axis of development of the 
Echinoderm-disk is in a difterent plane from that of the protozoid. 
Further, it is interesting to note that Sars, the discoverer of Bipinnaria, 
and the first careful describer of the life-history of Echinaster, has also, with- 
out any aid from other observers, been enabled to demonstrate the intermediate 
method of development occurring in Brachiolaria; while the independent 
investigations of A. Agassiz not only accord with his, but supply also some 
very desirable details touching the precise nature and relations of the same 
organism. 
Lastly, the young of Pteraster militaris, as described by Koren and Dani- 
elssen, though akin to that of Echinaster in its non-oceanic habit and develop- 
* Ueber den allgemeinen Plan in der Entwickelung der Echinodermen. 
Berlin, 1853 (p. 12). 
t A useful summary of our knowledge of the development of the Asteridea, 
with an expression of the author’s views, and ample references to original 
sources of information, is given by Wyville Thomson, in Nat. Hist. Rev. 1863 
(pp. 395-415). This article, together with Huxley’s Report on Muller’s re- 
searches, in Ann. & Mag. Nat. Hist. July 1851 (pp. 1-19), may be consulted 
for those details into which it does not become us here to enter. 
