250 Analytical Notices of Books. 
more nearly to the form and texture which they assume at their complete 
maturity, those which were before merely rudimentary now taking on 
their proper and distinctive character. Of the new organs that make 
their appearance in the third period the most important are the two livers, 
the brain, and the salivary glands. No new parts of any consequence are 
developed during the fourth period, nor is there any vestige, at its termi- 
nation, of internal sexual organs. 
The seventh section embraces the period between the bursting of the 
embryo from its shell, and the complete maturity of the animal. At the 
commencement of this period all its external organs are fully formed, 
but their outer coating is still extremely soft and flexible. It is not, 
however, necessary for it to proceed immediately in search of food, as it 
carries with it from the shell a portion of the vitellus, on which it can 
subsist until its coat becomes sufficiently hardened to admit of its moving 
from place to place with impunity. The following are the only outward 
changes that occur in it after quitting the shell. The legs increase in 
length more than in thickness ; and the same is the case with the antenne, 
the maxillz, and the spurious legs beneath the tail. These last acquire 
only at a very late period the long bristles which in the female serve for 
the attachment of the eggs. The anterior extremity of the thorax ac- 
quires two lateral spines in addition to the central one, which continues 
to increase in length. The pedicels of the eyes become thicker. The 
branchiz gradually increase in length, as do also, and with great rapidity, 
the little processes by means of which the animal absorbs the oxygen of 
the water. While the remains of the vitellus are being consumed, the body 
and tail gradually increase in length more than in breadth, and the latter 
also acquires a greater thickness. Lastly, the shell becomes firmer, and 
loses by slow degrees its parchment-like appearance by the addition of 
calcareous matter. In the mean time the number of its red points and 
streaks increases, and to these are added small blue spots which are most 
numerous on the upper surface and legs; the transparency of the 
outer coat for some time allowing the colours to be seen through it, pre- 
senting a beautifully variegated appearance. In the interior of the animal 
the changes are more important; but with the exception of the produc- 
tion and developement of the sexual organs, they consist, like those of the 
outer surface, in a gradual adaptation of parts already formed to their 
proper objects, 
