STRUCTURE OF CRAYFISH 32 1 



The animals swim backwards by powerful tail strokes, or 

 creep forwards on their " walking legs." Their life is 

 tolerably secure, but the frequent moultings during 

 adolescence are expensive and hazardous. When hatched 

 the young are like miniature adults ; for a time they chng 

 beneath the tail of the mother. 



External appearance. — The head and thorax are 

 covered by a continuous (cephalothoracic) shield ; the 

 abdomen shows obviously distinct segments movable upon 

 one another. As indicated by the appendages, there are 

 three groups of segments or metameres — five in the head, 

 eight in the thorax, six in the abdomen, as well as an un- 

 paired piece or telson on which the food canal ends. Each 

 of the nineteen segments bears a pair of appendages. 

 Among other external characters may be noticed the 

 stalked movable eyes, the two pairs o^ feelers, the mouth 

 with six pairs of appendages crowded round it, and the gills 

 under the side flaps of the thorax. 



((i) The external shell or cuticle, composed of 

 various strata of chitin, coloured with pig- 

 ments, hardened with lime salts ; 



(2) The ectoderm, epidermis, or hypodermis, 

 which makes and remakes the cuticle ; 



(3) An internal connective tissue layer or dermis, 

 with pigment, blood vessels, and nerves. 

 Internal to this lie the muscles. 



Between the rings and at the joints the cuticle contains 

 no Hme, and is therefore pliable. It is a layer not in itseL 

 living or cellular, made by the underlying living skin. As 

 it cannot expand, it has to be moulted periodically as long 

 as the animal continues to grow. The old husk becomes 

 thinner, a new one is formed beneath it, a split occurs 

 across the back just behind the shield, the animal with- 

 draws its cephalothorax and then i:ts abdomen, and an 

 empty but complete shell is left behind. The moulting is 

 preceded by an accumulation of glycogen in the tissues, and 

 this is probably utilised in the rapid growth which inter- 

 venes between the casting of the old and the hardening of 

 the new shell. 



How thorough the ecdysis or cuticle-casting is, may be appreciated 

 from the fact that the covering of the eyes, the hairs of the ears, the 

 lining of the fore-gut and hind-gut, the gastric mill, and the tendinous 



21 



The Body Wall 



consists of — 



