Description of a Species of Caligus. 239 
4. Posterior Thoracic Segment.—The legs attached to this 
Segment arise from the posterior part of the lateral surface. They 
are composed of four joints, which gradually diminish from the 
base to the apex. The basal joint is large, nearly cylindrical, and 
irregularly rounded at each extremity ; there is a short hairy seta 
at its apex. The second joint is scarcely half the diameter of the 
preceding. It gradually diminishes to a pointed apex, furnished 
with acurved spine. .The third joint is flat, and is articulated, by 
its obliquely truncated base, with the inner side of the preceding 
joint. There are two long sete on its inner apex, which are 
edged on two opposite sides with a pectinated membrane. There 
is a short pectinated appendage, projecting like an epaulette over 
the base of each of these sete, and also over the articulation of 
the following joint. This terminal joint is long, slender, and 
setiform. It has a row of short spines along its inner edge. 
5. Abdominal Segments.—The anterior abdominal segment is 
entirely destitute of any articulated appendages. Posteriorly, on 
each side, there is a broad lamellar sub-triangular organ, which, in 
the male, is much elongated, while in the female, excepting young 
individuals, it is very short. 
The appendages to the terminal joint have already been des- 
cribed. 
Change of Skin.—But few facts have come under our obser- 
vation respecting the change of skin; these few, however, appear 
quite peculiar and worthy of remark. 
When the time for shedding the old skin approaches, the inter- 
nal membrane, which is to form its new envelop, is very vari- 
ously folded into ridges throughout the whole body. In some 
parts, the ridges or folds are situated around the bases of the mus- 
cles, and enclose regular areas. These folds continue increasing 
in size, till the time of moulting. This process produces a sin- 
gular arrangement of the anterior portions of the inner shell, or, 
We may say, inner animal, as it affects the form of the included 
parts. The centre of the front margin of the internal shell is 
drawn inward and backward, as represented in fig. 23, Pl. V, in 
which ¢ d represents the outer margin, and c’n d’ the correspond- 
ing edge of the inner shell, e f the articulation in the old shell, 
and e’m f’ the corresponding articulation in the inner. 
These folds undoubtedly result from an increase of the animal 
within a shell too small to admit of its expansion. The internal 
