DECAPODA 385 



developed in the former ^roup and small and bent under the eephalo- 

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

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

 oHiers and chelate, fomiing the pinching claws (chelae) ; other periopods 

 also often chelate; egg^s 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 : 

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



usually present 1. Macbura 



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



2. Brachyuba 

 Suborder 1. ]\IACRURA.* 



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 havmg 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: 

 Ci Last pair of thoracic feet normal ; swimming fin present. 

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



scale large (Fig. 617) 1. Cabidea 



62 Burrowing marine animals of moderate size ; antennal scale usually 



absent 2. Th alassinidea 



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



nal scale, or none •• 3. Astacidea 



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



fin ; hermit crabs, etc 4. Anomuba 



Teibe 1. CARIDEA.t (Mackura 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 : 



Ci First 3 pairs of periopods not all chelate. 



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



chelate • 1. Crangonidae 



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



2. Pal^monidae 

 02 First 3 pairs of periopods chelate 3. Peneidae 



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

 F. H. Herrick, Mem. 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. 



