THE CRUSTACEA 117 



ment has one pair of jointed limbs, and therefore a segment having 

 more than one pair of limbs attached to it may be regarded as 

 composed of several segments fused together. In this segmentation 

 of the body the Crustacea fall naturally into three categories : (i) 

 the Entomostraca, in which the number of segments is indefinite ; 

 (2) the Malacostraca, in which may be counted nineteen segments, 

 exclusive of the terminal piece or telson and omitting the lateral 

 eyes ; and (3) the Leptostraca, including the single recent genus 

 Nebalia, in the abdomen of which two additional segments occur. 

 The Malacostraca includes the Crabs, Lobsters, Prawns, Hermit 

 Crabs, Sandhoppers, Woodlice, and their allies ; while the Ento- 

 mostraca comprises a vast assemblage of small Crustacea in which 

 the eyes are rarely borne on movable stalks, and of which the 

 so-called "Water-fleas" are a familiar example. 



In nearly all the Crustacea the young pass through a series 

 of metamorphoses after they quit the egg, and do not at first 

 resemble the final adult stage. Thus the young of the Common 

 Shore Crab (Carcinus mcenas), on escaping from the egg, does not 

 resemble its parents in the least. It has fixed eyes without eye- 

 stalks, a long body destitute of any appendages ; it has no walking 

 legs, but uses its maxillipedes, or jaw-feet, for purposes of loco- 

 motion, swimming with a jerky motion through the water. In 

 this, the zocea stage, as it is called, the baby Crab is a most comical- 

 looking little creature, with a great helmet-like head, crowned 

 by a long spine rather like an exaggerated dunce's cap, and a 

 second long spine or nose in front. The little thing swims actively 

 about in the sea, and, casting its skin from time to time, passes 

 through a gradual series of changes until it reaches the megahpa 

 stage, in which the eyes have become mounted on stalks, the ten 

 legs have developed, the body has broadened, and the tail short- 

 ened. Finally, it gives up the free-swimming life, comes close to 

 the floor of the sea, tucks up its abbreviated tail beneath the 

 body, and assumes the adult form. 



A Prawn belonging to the genus Peneus undergoes an even 

 more striking series of changes. It quits the egg as a tiny crea- 

 ture with an unsegmented body, a single eye in front, and three 

 pairs of swimming feet ; the first pair being simple, while the other 

 two pairs are branched. The mouth is covered by a helmet- 

 like hood. This stage is called the naiiplius, and after several 



