ARTHROPODS. 



125 



We shall recur again to these changes in the moths, and show how the 

 changes are not so abrupt as they seem. In the crabs and shrimps similar 

 modifications occur. The long-drawn-out embryo represented in Figure 

 112 does not apparently bear the slightest resemblance to the next figure 



Fig. 113. — Porcelain crab (Porcel- 

 lana) developed from the larva 

 shown in Figure 114. 



Fig. 112. -Young (Zoea) of the porcelain crab (Porcellana), greatly enlarged. 



but still it is the young of it. In the young the body is prolonged into 

 spines, which, while they add but little to the weight of the body, so 

 increase its size that it is not so easily swallowed by some of the other 

 animals which frequent the same waters. These 

 changes in shape, in both insects and Crustacea, 

 are accompanied by a molting or shedding of 

 the skin. The hardened external envelope of 

 the hody is rigid and does not stretch, while 

 its structure is such that it does not admit of 

 interstitial growth; but the animal is con- 

 stantly feeding, and with assimilation of food 

 there is a constant increase in size, and to 

 accommodate this the integument splits at 

 intervals, more or less regular, and the animal escapes from the opening, 

 and soon becomes clothed with a new skin larger than the old, which gradu- 

 ally acquires the hardness of the one which was lost, and then in turn is 

 cast off to be replaced by a new and still larger one. 



As we have said, these molts are frequently accom- 

 panied by changes of form sometimes enormous in ex- 

 tent, but these changes are really more gradual than 

 one would imagine were he to study merely the cast 

 skins. When studying the embryos under the micro- 

 scope, the naturalist sees the new shape gradually build- 

 ing itself up under the old skin, as is shown in the 

 adjacent cut, which represents the outline of one of 

 the. mouth parts of an embryo mud-crab, inside of which 

 the shaded portion shows the shape which will be assumed 

 after the next molt. 



The study of these metamorphoses is fraught with interest to the 

 thoughtful naturalist, for they show him a record of the descent of the 

 animals. The occurrence of this feature or that is an indication that 



Fig. 114.— One of the 

 mouth parts of an 

 embryo mud-crab, 

 showing the outline 

 of the skin soon to 

 be cast, and within 

 it the shape to be 

 assumed after the 

 molt. 



