CRA YFISH. 267 



secure, but the frequent moultings during adolescence are 

 expensive and hazardous. When hatched the young are 

 like miniature adults ; for a time they cling 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 unpaired 

 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 of 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 ; 



The BODY WALL 

 consists of 



(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 lime, and is therefore pliable. As a sacrificed product 

 of epidermic cells, it is dead and cannot expand. Hence, 

 as long as the animal continues to grow, periodic moulting 

 is necessary. The old husk becomes thinner, a new one is 

 formed beneath it, a split occurs across the back just 

 behind the shield, the animal withdraws its cephalothorax 

 and then its 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 intervenes between the casting of 

 the old and the hardening of the new she 11. 



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 

 inward prolongations of the cuticle to which some of the muscles are 

 attached, are all got rid of and renewed. The moults occur in the 

 warm months, eight times in the first year, five times in the second, 



