HIGHER CRUSTACEA OF NEW YORK CITY 151 



Order 6 ARTHROSTRACA 



Malacostraca in which the first, and in some cases the second 

 thoracic segment also, is fused with the head. The remaining seg- 

 ments are free, and there is no carapace. Eyes generally sessile or 

 without stalks. 



A large group, none of which attain very great dimensions. The 

 head and the first thoracic segment are indistinguishably fused to 

 form an incomplete cephalothorax, and the first thoracic appendage 

 is a maxilliped. The next seven thoracic segments are free and bear 

 legs, and the abdominal segments may or may not be fused. 



The large eggs are carried by the female in a brood pouch formed 

 by flat platelike appendages of certain thoracic legs. There is no 

 metamorphosis, the young being hatched in a form resembling the 

 parent. 



Divided into two suborders : a Amphipoda, b Isopoda. 



Suborder a AMPHIPODA 



Arthrostraca in which the body is usually compressed from side 

 to side. The gills are borne on the thoracic appendages. 



The larger suborder of the two. In addition to the above charac- 

 ters, it may be distinguished by the large abdomen, which, except 

 in the Caprellidae, where it is vestigial, is as long as the thorax and 

 has the segments free. 



Both pairs of antenna are present and either or both may be well 

 developed. Eyes sessile in all of our forms. The first two pairs 

 of thoracic legs, which are known as gnathopods, are frequently 

 large, specially in the males, and subchelate, the terminal joint being 

 bent back against the penultimate. The last three pairs are gener- 

 ally long and adapted for leaping. The abdominal appendages are 

 biramous and the first three pairs are long and delicate and function 

 as swimming organs. The last three are stouter and assist in leap- 

 ing and are known as uropods. 



To this suborder belong the " beach fleas," which are seen actively 

 skipping around on the sand or hiding, sometimes in great numbers, 

 under debris on the shore. Others are found under stones near 

 tide marks, or burrowing in the sand or living in tubes of their own 

 construction. Still others live in seaweeds or among eelgrass. 



