50 CRUSTACEA. 



In front of the abdomen there are seven distinct rings} 

 which carry seven pairs of appendages, and in front ofl 

 these is one pair of maxillipeds. The first ring of the [ 

 thorax is only represented by remnants of its ventral 

 and lateral portions, but bears the first pair of maxilli- 

 peds. The remnants are tucked away between the 

 cephalic shield and the second ring of the thorax, and 

 are entirely concealed from view until closely looked 

 for. These eight rings and their appendages form the 

 thoracic: region. In the Orchestia agilis,* a species 

 which lives in holes on the beach, the first thoracic 

 ring and the first pair of maxillipeds are clearly seen. 

 In this animal the last three pairs of swimmerets are 

 used for leaping, both the Orchestia and the (iam- 

 marus leap or swim head first, but while the former I 

 can stand upright, the latter, on account of its narrow 

 body, can only swim lying on its side or back down, 

 the appendages being brought closely together by the 

 lateral compression of the body. The first two limbs 

 of the thorax are used for clasping, a curious jaw being 

 formed by the bending back of the terminal section 

 against the next inner one. 



Attached to the bases of the thoracic: limbs are the 

 sacdike gills. The carapace is reduced to a small 

 head-shield, as in Idotrca, so that the thoracic: rings 

 are not covered, and, with the exception described, 

 may be easily counted. 



The head looks like one segment, but it carries two 

 pairs of antenna:, one pair of mandibles, and two pairs 



* Sec Annual Reports of the United States Fish Commission, 

 1871-72, PI. IV, Fig. 14. 



