154 INTRODUCTION TO ZOOLOGY chap. 



symmetrical, but unlike worms they have many pairs of well- 

 developed jointed legs, and each of these, as well as the head 

 and body, is protected by a hard, horny, and calcified sheath 

 ... or shell secreted by the skin below. This shell 

 is known as the exo-skeleton, and is peculiar in 

 being jointed with each joint of the legs, and in its seg- 

 mentation over the body wherever the need for bending 

 renders such a segmentation advantageous, the skin between 

 the segments being left soft and uncalcified. In this way the 

 whole body is efficiently protected, and at the same time the 

 legs are strengthened and so can not only support, but readily 

 carry along, a much larger body than is possible in less 

 specialised animals. Hence we find amongst those Arthropods 

 such as the crabs and lobsters, in which the protective shell 

 is most strongly developed, that the creatures reach a very 

 considerable size and yet can move rapidly. 



The different mode, however, in which the shell is formed, 

 and its increased complexity, are correlated with a dis- 

 advantage that does not occur amongst the Mollusca. A 

 snail keeps the same shell throughout life, merely adding to 

 the margin of it to meet the requirements of its growing body. 

 In an Arthropod such a simple method of increasing the size 

 of the shell is not possible, for it has been secreted from 

 the whole skin below, and is a dead product which, when 

 once formed and hardened by exposure to the air, is incapable 

 Moult of °^ growth or even of extension. When, therefore, 



Shell, the shell gets too tight for the body within, it is 

 or Ecdysis. bu^gt and thrown off entirely, and the soft-bodied 

 creature, which is now exposed, expands rapidly, before the 

 new shell, which has been secreted below the old one, has 

 had time to harden. Such a moult or ecdysis takes place 

 periodically at fairly frequent intervals in those Arthropods 

 which have thick calcareous shells and which increase in size 

 for several years. Professor J. A. Thomson says of the Crayfish 

 that "the moults occur in the warm months, eight times in 

 the first year, five in the second, two in the third, after which 

 the male moults twice and the female once a year, till the 

 uncertain limit of growth is reached." 



During the moulting period the animal is very defenceless, 

 and this time is usually spent in seclusion in some retired 

 spot. In Arthropods of the Insect type, where the "shell" 



