DECAPODA 385 



developed in the former group and small and bent under the cephalo- 

 thorax in the latter; 3 pairs of maxillipeds and 5 pairs of periopods 

 present; first pair of periopods (chelipeds) usually much larger than the 

 others and chelate, forming the pinching claws (chelae) ; other periopods 

 also often chelate ; eggs and sometimes the young carried on the pleopods : 

 about 6,000 species, mostly marine; the crayfish, certain crabs, and a few 

 others being fresh-water or terrestrial animals; 2 suborders. 

 Key to the suborders of Decapoda: 



a x Body more or less cylindrical and elongate; antennae long; tail fin 



usually present 1. MACBUBA 



a 2 Cephalothorax short and broad, with the abdomen bent under it ; crabs. 



2. BBACHYUBA 

 SUBORDER 1. MACRURA.* 



Body more or less cylindrical and elongate with a well-developed 

 abdomen, at the hinder end of which is usually a swimming fin formed 

 of the sixth pair of pleopods (uropods) and the telson; antennae well 

 developed and usually long, the first antenna having 2 or more flagella, 

 the second usually with an antennal scale; the young are born as nauplii 

 in Peneus and Lucifer, but in most other forms in a more advanced larval 

 stage: about 10 American families grouped in 4 tribes. 



Key to the tribes of Macrura : 



a t Last pair of thoracic feet normal ; swimming fin present. 

 &x Shrimps and prawns ; body rather small and transparent ; antennal 



scale large (Fig. 617) 1. CABIDEA 



6 2 Burrowing marine animals of moderate size; antennal scale usually 



absent 2. THALASSINIDEA 



6 3 Crayfish and lobsters ; body of moderate or large size, with small anten- 



nal scale, or none 3. ASTACIDEA 



a 2 Last pair of thoracic feet reduced and projecting upwards ; no swimming 



fin ; hermit crabs, etc 4. ANOMUBA 



TRIBE 1. CABIDEA. t (MACRURA NATANTIA.) 



Shrimps and prawns. Small forms with a compressed and more or 

 less transparent body; carapace smooth, without sutures and with a long 

 rostrum; antennal scale large; thoracic legs usually long and delicate: 

 about 17 families and several hundred species. 



Key to the families of Caridea here described : 



o t First 3 pairs of periopods not all chelate. 

 &! Second pair of periopods only chelate ; first pair very stout and sub- 



chelate 1. CBANGONIDAE 



& 2 First 2 pairs of periopods usually chelate ; first antennae with 3 flagella. ^ 



2. PAL^MONIDAE 

 o 2 First 3 pairs of periopods chelate 3. PENEIDAE 



See "Embryology and Metamorphosis of the Macroura," by W. K. Brooks and 

 F. H. Herrick, Mern. Nat. Acad. Sci., Vol. 5, 1892. 



t See "Synopsis of the Caridea of North America," by J. S. Kingsley, Am. Nat., 

 Vol. 33, p. 709, 1899. 



