SUBPHYLUM CRUSTACEA 



221 



The crayfish, which has already been described as a type, 

 belongs to the most highly developed class, the Malacostraca, 

 the most prominent features of which are the stalked compound 

 eyes and the large carapace covering the thorax. This class 

 includes the lobsters, crabs, prawns, and shrimps, and also the 

 best-known terrestrial Crustacea, the woodlice of the Order 

 Isopoda. Other classes are the Branchiopoda, characterised 



Fig. 155. — Cyclops. 



abA, First abdominal segment ; a^.i, antennule ; at 2, antenna ; c.f., caudal fork ; cph., cephalothorax (fused 

 head and first two thoracic segments) ; e.s., egg sac ; eye (single and median) ; g., ahmentary canal ; 

 gen.op., genital opening ; t., telson ; th.^, thfi, third and sixth thoracic segments. 



In comparing this crustacean with the crayfish, note the absence of proventriculus, paired eyes, 

 vuropods, and carapace, the presence of median eye and caudal fork, and the difference in the 

 number of segments. 



by fiat abdominal appendages (phyllopodia) , e.g. Daphnia, 

 the water flea ; the Ostracoda, with a bivalve carapace, e.g. the 

 freshwater Cypris ; the Copepoda, with no abdominal appendages, 

 which include the free-living freshwater Cyclops, and a number 

 of interesting parasites such as Argulus, the carp louse ; and the 

 Cirripedia, which are sedentary when adult and include the 

 barnacles such as Lepas and Balanus, and extreme parasites 

 such as Sacculina, which, living partly attached to the underside 

 of the intestine of a crab, and partly beneath its abdomen, 

 sends suckers which penetrate throughout the body of its host. 



