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 a curved 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 setse 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 setee, 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 Skifi. — 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 aff'ects 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, PI. V, in 

 which c d represents the outer margin, and & 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 



