TYPE CRUSTACEA. 413 



antermules are small and they and the eyes can be partially 

 concealed in a groove on the anterior edge of the carapace. 

 The abdominal appendages, with the exception of the anterior 

 one or two pairs which are adapted for copulation, are ab- 

 sent in the males, while the females generally possess four 

 pairs, to which the ova are attached. 



The gills are generally few in number, except in Porcel- 

 lana and some allied forms, and are usually lamellate in form. 

 While essentially marine in habit, the Brachyura are fre- 

 quently more or less terrestrial, the sand-crabs, Ocyjjoda, and 

 the fiddler-crabs, Gelasimus, living in holes in the sand just 

 above high-tide mark, while the land-crabs, Gecarcinus, of the 

 tropics may live some distance from the sea, migrating to it 

 in armies during the breeding-season. A few forms, such as 

 the genus Telphusa, are aquatic. 



III. SUBCLASS ARTHROSTRACA. 



The Arthrostraca, with the exception of the small group 

 of the Anisopoda, are destitute of a carapace, and the tho- 

 racic appendages, with the exception of the first pair, are 

 jointed walking-limbs lacking an exopodite. The anterior, 

 or in some cases the anterior two thoracic segments fuse 

 with the head, the appendages of these segments differing 

 from those of the free segments, being modified to assist in 

 the process of mastication, whence they are termed maxilli- 

 peds. The abdomen is composed of six segments provided 

 with appendages, and of a terminal telsou ; occasionally the va- 

 rious segments fuse together, and in Some forms the abdomen 

 is reduced to a small unsegmeuted structure. Platelike ap- 

 pendages attached to the basal joints of some of the thoracic 

 limbs form by their meeting and overlapping a brood-pouch 

 in which the ova undergo their development. 



The lateral compound eyes are not, except in Tanais, sup- 

 ported on stalks, a characteristic which has suggested the 

 term Edriophtlialmata sometimes applied to the group. 



1. Order Anisopoda. 



The Anisopoda are exclusively marine forms in which the 

 two anterior thoracic segments are fused with the head and 



