PHYLUM ARTHROPODA 



587 



no trace of crustacean organisation, and it is only bj^ the stud}- of 

 development that its true systematic position can be guessed. 



In the division Eucarida, the Eiipliansi(ii:c<i. (Fig. 472) are 

 pelagic forms in which none of the thoracic appendages are modi- 

 lied so as to take the form of maxillipedes, and in which there 

 is only a single series of branchiae (podobranchs). 



Amongst the Decapoda are included nearly all the largest and 

 most familiar Crustacea — the Prawns and .Shrimps, Lobsters, Cray- 

 fishes, and Crabs. The cephalothorax is always completely covered 

 by the carapace. The three anterior pairs of thoracic appendages 

 are modified into maxillipedes, which retain the original biramous 

 character, but the five posterior pairs are enlarged, and form legs. 



2. A: 



4. Cryptoniscus 



Fig. 464, — Zsopoda. 5,a, entire animal ; h, posterior end with attaehed male (m) ; h,a, larva ; 

 h, adult female. (After Cuvier, Claus, and Gerataecker.) 



which are always — except as an individual variation — devoid of 

 exopodites in the adult. 



In the Shrimps and Prawns (Fig. 465) the body is compressed, 

 and the exoskeleton is not calcified. The abdomen is very large 

 in proportion to the cephalothorax, and has a peculiar bend close 

 to its junction with the thorax. The legs are very slender, are 

 used for swimming, not walking, and sometimes one pair, sometimes 

 another, is enlarged to form the chelipeds. The rostrum is large 

 — sometimes longer than the rest of the carapace — and the eye- 

 stalks, antennae, and legs may attain extraordinary dimensions. 



The Lobsters and fresh-water Crayfishes agree with Astacus in 

 all essential details, but the sea-Crayfishes {Palinurus) present some 

 striking modifications. There are no chelae, the legs all ending in 

 simple claws : the antennae are of immense size, and their proximal 

 segments are fused with one another and with the carapace, quite 

 crowding out the epistoma : the rostrum is reduced, or even 

 vestigial, and the pleopods are very broad and fin-like. In Scyllarus 

 (Fig. 466) and its allies the body is broad and depressed, the bases 



