On the present state of Zoology. 7 



ly so called, are sufficiently great to mislead the inexperienced naturalist. 

 Equally important are those changes which have been traced with so 

 much attention and care by Duges in the case of the Acari* and 

 which have thus accounted for the apparent anomaly of hexapod and 

 octopod forms coexisting in the same group. But there are other 

 classes in which the metamorphosis is more complete, and attended 

 by such an entire alteration of habits and economy, that by no pos- 

 sible a priori reasoning could we have been led to consider the young 

 and adult states of such animals, if seen separately, as pertaining to 

 the same species. Who would have suspected that the sluggish Bar- 

 nacle, immoveably fixed to some rock or other marine substance, had 

 ever enjoyed a free independent existence, swimming rapidly in the 

 sea under the form of a small bivalve crustacean ? Still less who 

 would have anticipated the nature of those changes which attend the 

 early development of the compound Ascidice ? animals appearing at 

 birth as separate individuals, and endued with the power of locomo- 

 tion, uniting afterwards to form one common inert mass ! — Yet these 

 are the striking discoveries which have been made known of late years 

 by different observers, and the accuracy of which there appears no 

 reasonable ground for doubting. Now it will hardly be questioned 

 whether it be important for the naturalist to be acquainted with 

 these changes occurring in certain animals. It is obvious that, except 

 he be, he will be constantly mistaking the immature states of such 

 animals for distinct species, or perhaps be even referring them to dis- 

 tinct genera. We know that such errors in fact have occurred over 

 and over again. Thus the larvae of the Acori were, until the re- 

 searches of Duges, regarded as permanent forms, and made to consti- 

 tute a peculiar family in that group comprising several genera. The 

 6ame was the case formerly with some of the Entomostraca. It is 

 also probable that many of the minute animals, which now stand in 

 our systems as distinct forms, are only the first states of some of the 

 higher ones, in which the existence of metamorphosis remains to be 

 discovered.-)- But it is not merely to avoid the overmultiplication of 

 genera and species that this acquaintance with animals at different 

 periods of their growth is wanted. It is necessary in order to obtain 

 correct views respecting their affinities. There was a time when, to 

 attain this object, it was thought sufficient to look only to the per- 

 fect state of the species, and scarcely any notice was taken of the 



* Ann. des Sci. Nat. 1834, torn. i. pp. 5 and 144; and torn. ii. p. 18. 



f Milne- Edwards thinks it probable that some of the Cercarice are only the 



young of the compound Ascidia during the first stage of their existence See 



Lamarck's Hist. Nat. des An. sans Vert. (2d edit.) torn. i. p 428. 



