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 JEntomostraca, or the Lernceae to the 
siphonostomous 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.f 
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 t >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. Tratis. 1835, p. 301. 
