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CLASS CRUSTACEA. 



Mention the Distinctive Features of the Crustacea. 



The body is usually divided into three distinct regions : 

 head and thorax (frequently consolidated) and abdomen. 

 The head has two pairs of feelers or antennae, and three 

 or more pairs of gnathites or jaws ; the thorax bears the 

 principal appendages of locomotion, and the abdomen often 

 has appendages. The appendages are modifications of a 

 common type which has three parts : a two-jointed basal 

 piece (the protopodite) to which gills may be attached 

 and which bears two branches (outer exopodite and inner 

 endopodite). The exoskeleton consists of chitin impreg- 

 nated with lime salts. Respiration is by gills or through 

 the skin. 



The floating population or " plankton " of the sea and 

 fresh waters largely consists of Crustaceans (adults and 

 larvae) which are pelagic, occurring in myriads at or near 

 the surface ; and these (especially Copepods) are the prin- 

 cipal food of Fishes. 



Where is Sacculina found ? Give an Account of its Life- 

 History. 



Sacculina carcini is a parasite of the Shore-crab ( Carcinus 

 maenas). It begins life as a free-swimming Nauplius, a. 

 tiny oval larva which has three pairs of limbs and a dorsal 

 shield with two " frontal horns." The Nauplius changes 

 into a Cypris, which has a bivalved carapace and two- 

 antennae with prehensile hooks. This Cypris fastens on 

 to a young crab, fixing the hook of one antenna into the- 

 base of one of the crab's " hairs " or setae. Then it casts 

 off the greater part of its body ; and the remaining small 

 nucleus portion passes through the hollow antenna into 

 the body of the crab and is carried by the blood to the 

 outer wall of the intestine : there it becomes attached 

 and sends out ever-spreading " roots." As its bulk increases, 

 this " Sacculina interna " moves down the gut ; and when 

 it comes to where the abdomen bends abruptly in upon 

 the thorax, it causes a hole in the integument through 



