OR OSSEOUS SYSTEM. 41 



above, they form an inferior median canal to protect the 

 nerves and ganglia of this part of the trunk. There are 

 most frequently five pairs of legs, which like the exte- 

 rior pairs of jaws, have branchiae attached to their bases. 

 In the female of the decapods, we observe the round 

 opening of the vulva in each haunch of the middle or 

 third pair of legs ; and in the male the open sheath of 

 the penis is seen in each haunch of the fifth or posterior 

 pair of legs. The front pair of legs are generally the most 

 powerful, and have the last tarsal joint (/, /,) inserted high, 

 and opposable to the penultimate articulation (k, k). The 

 segments of the post-abdomen (m, n,) are moveable, and 

 sometimes extend backwards in a line with the cephalo- 

 thorax, or are short, and fold in beneath that part. Below 

 the middle of the last small segment, is the opening of 

 the anus, and the two last segments of the post abdo- 

 men have their lateral appendices (p, o,) in form of flat 

 expanded caudal plates for swimming and for protecting, 

 when folded inwards, the delicate inferior parts of the 

 trunk, or the ova in the female. On the lower surface 

 of the post-abdominal segments are attached the false 

 feet, to which the ova are affixed after their discharge 

 from the two ovaries, and from which the branchiae are 

 commonly suspended in the inferior orders of Crustacea. 

 The chief differences in the skeletons of this class arise 

 from the size, and the forms assumed by the convertible 

 lateral appendices of the segments, and the extent to 

 which the segments are developed or anchylosed. This 

 solid crust, forming the skeleton of Crustacea, is thrown off 

 periodically from every part of the trunk and even its most 

 delicate appendices. This is done by the animal first de- 

 taching the cutis and muscles from the inner surface of 

 the old shell, then excreting from the surface of the cutis 

 a new layer of epidermis, then a deposit of colouring 

 matter, and within this the calcareous materials of the new 

 shell, the old having been broken off in detached pieces 

 in succession from all parts of the body. 



