8 On the present state of Zoology. 



previous stages through which it had passed. This idea, however, is 

 in a great measure abandoned. It is now ascertained that the changes 

 of structure experienced by different animals are all the result of cer- 

 tain fixed laws, closely connected with those which regulate their af- 

 finities. Genera which are dissimilar in their adult states sometimes 

 resemble one another during the first periods of their development, 

 thus indicating a relationship which would escape our notice except 

 we were made acquainted with their early history.* It was not until 

 their metamorphosis had been detected, that the Cirripeda were fully 

 ascertained to be allied to the Entomostraca, or the Lernceos to the 

 sipJionostomous Crustacea. 



Another point of no less importance to be attended to than the 

 study of animals at different periods of their existence, is the study 

 of their whole structure. We need only observe how imperfect our 

 arrangement of the Mollusca was, so long as conchologists contented 

 themselves with the knowledge of shells, apart from all regard to the 

 nature of their animal inhabitants ; or how far we are still removed 

 from understanding the affinities of several other groups, of whose 

 structure we know little beyond the external form. — In all such cases 

 we are carried away by partial resemblances, and led to attach an 

 undue value to organs exercising only a subordinate function in the 

 economy. Thus the characters afforded by shells are not necessarily 

 in direct connection with those derived from the internal organiza- 

 tion. Several instances to the contrary have been adduced by Mr 

 Gray in a paper lately published in the Philosophical Transactions.-]- 

 The most remarkable are those of the genera Patella and Lottia. It 

 is observed that in these genera the shells are so perfectly alike, that 

 after, a long-continued study of numerous species of each genus, Mr 

 Gray cannot find any character by which they can be distinguished 

 with certainty, yet their animals are so extremely dissimilar,, as to be 

 referable to two very different orders of Mollusca. The Zoophytes 

 or Polypi have been subjected to the same misarrangement as the 

 Mollusca, owing to attention having been given almost exclusively 

 to the nature of the calcareous covering. It was thought by La- 

 mouroux, that this alone was sufficient to serve as the basis of their 

 classification. It is now found that the included animals exhibit very 



* It would lead us too much into detail, or we might here allude to those 

 beautiful generalizations lately established by Milne-Edwards with regard to the 



changes of form which occur before and after birth in the Crustacea See his 



memoir in the Ann. des Sci. Nat. already alluded to ; also a Report on that 

 memoir by M. Isidore Geoffroy St Hilaire. Id. 1833, torn. xxx. p. 360. 



f Phil. Trans. 1835, p. 301. 



