HIGHER CRUSTACEA OF NEW YORK CITY 1 27 



CATALOGUE 



The Crustacea are divided into two subclasses : I Entomostraca, 



2 Malacostraca. 



Subclass I ENTOMOSTRACA 



Crustacea of comparatively simple organization and usually of 

 small size. Except in the parasitic forms and the barnacles the 

 typical crustacean number of head appendages are present, but the 

 thoracic and abdominal appendages are very variable in number and 

 are little differentiated. A large carapace is frequently present, 

 being sometimes in the form of a bivalved shell. The young usually 

 hatch in a very primitive, free-swimming stage known as the 

 nauplius. 



But little has been done on this subclass in New York ; and it is 

 still far too early to attempt a catalogue of the species. 



Subclass 2 MALACOSTRACA 



Crustacea, usually of considerable size and more highly organized 

 than those of the preceding subclass. The number of segments is 

 constant, there being (except in one order, not represented in New 

 York) a thorax of eight segments and an abdomen of seven. In all 

 the orders represented here, one or more of the thoracic segments 

 are fused with the head to form a cephalothorax, which in some of 

 the orders is covered by a carapace. The thoracic appendages are 

 more highly differentiated than in the Entomostraca and are often 

 strikingly modified in connection with their various functions. One 

 or more pairs of them usually act as accessory mouth parts and are 

 known as maxillipeds. 



With but few exceptions, the nauplius stage, in which the young 

 of the Entomostraca hatch, is passed through in the egg, but except 

 in one order, the young- hatch in a form very unlike the parent and 

 thus undergo a metamorphosis. 



The members of this group are more familiar to the ordinary 

 observer, as it includes, among others, the crabs, lobsters, crayfish, 

 the beach fleas and the sow-bugs. 



