METAZOA CRUSTACEA. 389 



One of the external parasitic Crustacea is represented 

 by the whale louse, Cyamus ceti, Latr. (No. 832). Its 

 flattened body is reduced to a few segments. Most of the 

 appendages are provided with hooks whereby the crusta- 

 cean fastens itself upon the whale. This is the case with 

 the arms which have no claws, but the movable jaw is 

 modified into a sharp curved hook. The third and fourth 

 apparent segments bear long tubular branchiae. 



Isopoda. Idotea wossnessenski Brundt (No. 833), has 

 the body flattened from above and divided into seven dis- 

 tinct thoracic segments and an indistinctly segmented 

 abdomen. The head is provided with a pair of small 

 sessile eyes and two pairs of antennae, one pair being 

 much smaller than the other. Then follow seven pairs of 

 similar jointed feet placed far apart on the lateral edges 

 of the ventral side and adapted for walking. Between 

 these legs the respiratory leaves are snugly folded over 

 the eggs. The abdomen is supplied with leaf-like swim- 

 merets and the first pair are modified into a cover for the 

 other. 



The segments of the depressed body are similar from 

 one end to the other, there being slight differentiation 

 between the thoracic and the abdominal regions. 



The head with its pair of sessile eyes is inconspicuous 

 but it carries one pair of long antennae. The thoracic 

 region has three pairs of jointed feet followed by two 

 segments without feet, and these in turn are succeeded 

 by three segments with feet. The respiratory organs are 

 leaf-like and enclose the young larvae. The thoracic 

 appendages are followed by several pairs of abdominal 

 leaf-like organs. 



The fresh-water Asellus communis Say (No. 834) is a 

 little crustacean with a long, narrow, depressed body 

 composed of similar segments, none of which is covered 

 by a carapace. The head is provided with two pairs of 

 antennae. The thorax consists of seven segments and 

 carries seven pairs of walking-feet. The terminal seg- 



