254 CRUSTACEA. 



and there in England and Ireland, and is common on the 

 Continent. It is absent from districts where the water 

 contains little lime. The food is very varied — from roots 

 to water rats ; cannibalism also occurs. The animals swim 

 backwards by powerful tail strokes, or creep forwards on 

 their "walking legs." Their life is tolerably secure, but 

 frequent moultings occur which 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 (cephalo- 

 thoracic) 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. (According to some authorities there are twelve 

 segments in the cephalothorax, and seven in the abdomen.) 

 Each of the nineteen segments bears a pair of append- 

 ages. 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, the gills 

 under the side flaps of the thorax, and the varied post-oral 

 appendages. 



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

 various strata of chitin, coloured with pig- 

 ments, hardened with lime salts ; 

 The Body Wall J (2) The ectoderm, epidermis, or hypodemiis, 

 consists of: — | 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. 



