6 On the present state of Zoology. 
easily accessible, and whose habits and economy are in consequence 
almost entirely concealed from view. We find others possessed of 
such a delicate and fragile structure, that all attempt to convey them 
from their native spots, or to preserve them for future examination 
and comparison, is rendered hopeless. These remarks are especially 
applicable to the marine Invertebrata, with which our acquaintance 
is as yet comparatively slight. How many multitudes of species 
must there be immersed in the profound recesses of a deep ocean, 
where no net or line ever reached. How many more, which, 
if known, have been seen only at a casual glance, or at one period 
of their lives, leaving- us in utter ignorance of the details of their 
history, and of the changes through which they pass in their advance- 
ment to maturity. How many, like the Acalepha, can hardly be re- 
moved from their native element, without entailing the entire disor- 
ganization of their substance ; and if there be others which we suc- 
ceed in preserving in our cabinets, how impossible it is, in most in- 
stances, to effect the preservation of those parts, on which depend 
perhaps their most important characters, and in ignorance of which 
we may adopt the most erroneous views respecting their structure 
and affinities. 
We shall be able to appreciate more fully the impediments to the 
progress of Zoology arising from the sources last mentioned, if we 
bear in mind two points of great consequence to be remembered. 
One is the necessity of studying animals throughout their whole ex- 
istence in order to become possessed of their true history. Not only 
do all animals differ more or less in their young and their adult states, 
but many, probably more than we are at present aware of, undergo 
such great changes of structure, that, unless viewed in their transi- 
tion forms, it were almost impossible to identify them as the same 
species. There is no occasion to allude to the Batrachian Reptiles 
or to the true Insects, in which these changes have been noticed from 
the earliest times. But we may draw attention to the circumstance 
that metamorphosis, to a greater or less extent, has been recently de- 
tected in several groups in which it was not supposed formerly to 
exist, and that therefore we stand in need of further observation to 
say in what others it may not also occur. Without insisting on the 
universality of this phenomenon in the class Crustacea, which can- 
not be considered as established, and of which we shall have to speak 
further presently, we may allude to those marked changes of form 
which have been noticed in certain groups of that class by M. Milne-Ed- 
wards,* and which, if not amounting to actual metamorphosis proper- 
* Ann. des Sci. Nat. 1835, torn. iii. p. 321. 
