On the present state of Zoology. 1 1 
ticulars connected with the structure and economy of these animals.* 
We can only state here, that, according- to the researches of that 
gentleman, the former offers three very distinct modifications, of 
which two only belong- properly to the type characteristic of the Ra- 
diata. In these two he has traced an organization gradually becom- 
ing more and more simple from the Actinia and Heroes down to the 
Hydrce. The third series opens a passage from the compound Asci- 
dice to several of the Infusoria Polygastrica of Ehrenberg. 
When speaking of the importance of studying animals at different 
periods of their growth, we pointed out certain groups in which strik- 
ing changes of form had been observed to take place, approaching 
more or less to the nature of metamorphosis. The desire to investi- 
gate further these structural changes forms another prominent feature 
in the memoirs of the present day. The Cirripeda and Crustacea 
have excited particular interest on this head. It is well known that 
Mr Thompson was the first to announce the existence of metamor- 
phosis in each of these two classes, a fact which has been since con- 
firmed by others as regards the Cirripeda, but which is much doubt- 
ed by naturalists as regards the Crustacea. During the past year, 
two memoirs have appeared from the pen of this gentleman devoted 
respectively to these two subjects. The object of the first f is to 
make known the discovery that the Lepades are, like the JSalani (to 
which last his former researches alone extended), natatory Crustacea 
during the first stage of their existence, but of a totally different struc- 
ture. The larva is said to resemble that of the genus Cyclops, or 
still more closely that of the Argulus armiger of Latreille. Mr 
Thompson is uncertain how long the Lepades remain in this their 
first or free state, but thinks that it is for a longer or shorter period, 
according as they may happen to meet with a substance adapted to 
their habits, to which they can attach themselves. In his second 
memoir, Mr Thompson treats of the Crustacea ; and his object is, not 
merely to confirm his former statements respecting the existence of 
metamorphosis in this class, but to announce the startling discovery 
that these animals, before attaining to their perfect form, undergo 
two marked transformations, analogous to those experienced by the 
true insects, f It will be remembered that the first state in which 
* See L'Institut, 1835, pp. 10 and 152. Also Ann. des Sci. Nat. (Second 
series.) torn. iv. p. 321. 
f Discovery of the Metamorphosis in the second type of the Cirripcdes, viz. 
the Lepades, completing the natural history of these singular animals, and con- 
firming their affinity with the Crustacea Phil. Trans. 1835, p. 355. 
{ On the double Metamorphosis in the Decapodous Crustacea, exemplified in 
Cancer Mcenas, Linn Phil. Trans. 1835, p. 359. 
