238 Description of a /Species of Caligus. 



they diverge from their insertion and extend to the base of the 

 apron, (fig. 18. ) Here they are united by a slender osseous process, 

 which forms the upper hmits of the sternum of the apron. They 

 continue on, making at first one or two irregular curves, and form 

 also the lateral boundaries of this sternum. By this remarkable 

 arrangement, the articulation of the sternum of the large natatory 

 legs is rendered sufficiently firm for their powerful action in the 

 motions of the animal. 



The third pair of legs has been already stated to be expanded 

 into the form of an apron, forming a broad lamellar appendage to 

 the cephalo-thoracic portion of the body. This apron is composed 

 of the same parts as the natatory last described, and there is an 

 almost perfect coincidence in the number and nature of the ap- 

 pendages. 



The sternal piece is very wide and lamellar. On its posterior 

 margin there is a broad membranous expansion, identical in struc- 

 ture and position with that appended to the preceding sternum. 

 The portion of the apron corresponding to the small first joint of 

 the natatory leg, is very narrow, and has very imperfectly defined 

 limits ; we see an indication of its presence in the single pinnula, 

 behind, adjacent to the sternum.* The second joint is expanded 

 into a broad, irregular trapezoidal figure, with concave sides ex- 

 cepting its posterior margin. Like the same joint in the natatory, 

 it is furnished with a broad thin membrane posteriorly, and a 

 jointed appendage provided with pinnulse. The first joint of this 

 appendage is quite small, and bears a single pinnula as in the per- 

 fect leg ; the remaining portion is circular and is furnished with 

 six pinnulae. We find the analogue of the third joint of the 

 natatory in a broad nearly circular plate, which is the lateral termi- 

 nation of the apron ; it is connected with the basal portion by an 

 indistinct suture. The posterior margin of this joint, like the 

 same joint above, bears a ciliated leaf-like expansion. The anal- 

 ogy of the parts is still farther apparent in the strong articulated 

 spine and pinnula attached to this joint, and in the two small termi- 

 nal joints, furnished with pinnalas ; the first with a single pinnula 

 and a spine at its apex, and the second with four pinnulce, and two 

 short apical spines. 



* The corresponding parts of the apron, and the second pair of natatories, are in- 

 dicated in the figure by the similar numbers in them. 



