282 PHYL UM AR THROPODA. 



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

 ably 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 ! (2) The ectoderm, epidermis, or hypodermis, 

 consists of I 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. It is a layer not in itself 

 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 its abdomen, and an 

 empty but complete shell is left behind. The moulting is 

 preceded by an accumulation ofglycogen 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 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 



