XI 



PHYLUM ARTHROPODA 



537 



(ex), thus retaining the primitive biramous or "split-footed" 

 form which is lost in the Decapoda. The first five pleopods 

 are large in the male, small in the female : the sixth is a uropod, 



. My Sis OCUlata. end. endopodite ; en. enopodite ; ot. otocyst. (After Gerstaecker.) 



assists the telson in the formation of the characteristic 

 lalacostracan tail-fin : there is no trace of the entomostracan 

 caudal styles. 



Amongst the Decapoda are included nearly all the largest and 

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

 fishes, and Crabs. The cephalo-thorax 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, 

 which are always except as an individual variation devoid of 

 exojtodites in the adult. 



In the Prawns (Fig. 424, ./) the body is compressed, and the exo- 

 skeleton is not calcified. The abdomen is very large in proportion 

 to the cephalo-thorax, and has a peculiar bend close to its junction 

 with the thorax. The legs are very slender, 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 both 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 (Pcdinurus) present some 

 striking modifications. There are no chela3, the legs all ending in 

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

 segments a t re 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 



