382 CRUSTACEA 



B. hippolytes Kroyer. Body of female 8 mm. long and 7 mm. wide; 

 of male 3 mm. long and 1 mm. wide: circumpolar, extending to Boston 

 and to Puget Sound; on the gills of Pandalus and other prawns. 



Division 3. THORACOSTRACA.* 



Malacostraca often of large size in which 3 or more of the thoracic 

 somites are fused with the head, and the cephalothorax thus formed is 

 covered with a carapace; projecting forwards from the anterior end of 

 the carapace in most species is the spike-like rostrum; eyes at the end of 

 movable stalks or peduncles (except in the Cumacea) ; the sixth pair of 

 pleopods (uropods) together with the last body segment (telson) forms, 

 except in the case of the crabs, a swimming fin, by striking which vigor- 

 ously beneath the body the animal propels itself rapidly backwards; 

 the eggs and sometimes the young usually carried beneath the abdomen 

 attached to the pleopods, the young in most forms passing through a 

 metamorphosis before attaining the form of the parents: 4 orders. 



Key to the orders of Thoracostraca: 



a^ Carapace does not cover the entire thorax. 



6i Thoracic appendages all biramose 1. Schizopoda 



?>2 Thoracic appendages not all biramose. 

 Ci Abdomen large and wider than the small cephalothorax. , .2. Stomatopoda 



Ca Abdomen narrow 3. Cumacea 



O2 Carapace covers the entire thorax 4. Decapoda 



Order 1. SCHIZOPODA. 



Body elongate and usually more or less transparent and with a thin 

 carapace which covers nearly all of the thorax; the 8 thoracic feet may 

 all support gills and are biramose, the anterior 2 pairs being slightly 

 modified to form maxillipeds; eggs carried beneath the thorax as in the 

 Arthrostraca ; young born in some species as nauplii: 3 families and 

 about 300 species, mostly marine; 11 American species. 



Family MYSIDAE. 



No gills present; first 2 pairs of thoracic appendages (maxillipeds) 

 shorter than the following 6; abdominal appendages often rudimentary 

 in female; the endopodites of the uropods bear each an auditoiy sac; 

 2 to 7 pairs of marsupial plates beneath the thorax within which the 

 young develop: 21 genera and 90 species, mostly marine. 



1. Mysis Latreille. Body laterally compressed; fourth pair of ab- 

 dominal appendages in male are long stilets; antennal scale long: often 

 in swarms in the North Atlantic; 23 species, 4 American, 1 in fresh 

 water. 



• See "The Stalk-eyed Crustaceans of the Atlantic Coast," etc., by S. I. Smith, 

 Trans. Conn. Acad., Vol. 5, p. 27. 



