572 



ZOOLOGY 



SECT. 



" Pill-bugs " (Armadillidium, 2) have the habit of rolling them- 

 selves up into a ball when disturbed. Cymoihoa and its allies are 

 large species (6-8 cm. in length) parasitic in the mouths of Fishes, 

 where they hold on to the mucous membrane with their short, 

 clawed legs : their mouth-parts are often modified for sucking. In 

 the Bopyrini, found in the gill-cavities of various Crustacea, para- 

 sitism is accompanied by great degeneration and asymmetry, 

 as well as by a notable degree of sexual dimorphism, the males 

 (3 b, m) being very small and permanently attached to the bodies of 

 the females. Lastly, in Cryptoniscus, parasitic on Crabs, the adult 

 female (4 6) has no trace of crustacean organisation, and it is only 

 by the study of development that its true systematic position can be 

 guessed. 



In the division Eucarida, the EupJiausiacea (Fig. 479) are pelagic 

 forms in which none of the thoracic appendages are modified so 

 as to take the form of maxillipedes, and in which there is only 



a single series of 

 branchiae (podo- 

 branchs). 



Amongst the 

 Decapoda are in- 

 cluded nearly all 

 the largest and 

 most familiar 

 Crustacea the 

 Prawns and 

 Shrimps, Lob- 

 sters, Crayfishes, 

 and Crabs. The 

 >r end with cephalothorax is 



(After Cuvier, alwayg c Q m . 



pletely 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 

 exopodites in the adult. 



In the Shrimps and Prawns (Fig. 472) 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, ar^e 

 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 (Pcdinurus) present some 



2. Armadillidium. 



3. Gyge. 



4. Cryptoniscus. 



FIG. 471. Isopoda. 3, a, entire animal ; 6, _ 

 attached male (m) ; 4, a, larva ; b, adult female. 

 Claus, and Gerstaecker.) 



