Chapter IV.— MOLTING. 



Molting is an incident and expression of growth. The crustacean does not "grow 

 by molting," as is sometimes said, but it molts because it has grown. It has outgrown 

 its inelastic shell, which is cast off in one piece, normally without a break in any of 

 its hard parts. Other animals molt or shed a part of their cuticle and its products, but 

 nowhere is the process so striking, so abrupt, or so critical as in the higher Crustacea. 

 In these animals the span of life from infancy to old age and death may be divided into 

 a series of stages, varying in length, each stage-period of life culminating in a molt. 



Any influence which retards growth or unduly taxes the vital energies prolongs 

 this period, and conversely the more vigorous and the more rapid the growth the shorter 

 the interval between molts. Shortly after molting the body increases in size, probably 

 in part through the absorption of water, but this expansion should be distinguished 

 from the change that has already taken place, which is due to cellular growth, and is 

 the primary cause of the molt. Thus in molting the animal parts with its old shell or 

 epidermic exoskeleton at one stroke, and presently attains to greater size. 



Molting begins on the second day after hatching and lasts throughout life or at 

 least as long as there is any growth. The first three molts are passed in from 12 to 15 

 days. From first to last the cuticle is cast as one piece (excepting only the gastro- 

 liths), the animal escaping through a rent of the membrane between the tail and back. 

 In healthy young animals molting lasts but a few minutes, but at all times the process 

 is critical and it is frequently fatal. It often leads to the distortion or the loss of limbs 

 and to a variety of deformities such as duplications of a limb or of its parts. 



It is difficult to avoid repetition in dealing with the molting process since it has 

 modified the habits of the animal at so many points, but we shall now consider the 

 subject in regard to the adult animal as a whole. In order to understand the process 

 it will be necessary to examine the structure of the shell and of the soft skin, of which 

 the former is a product. 



THE SKIN AND SHELL. 



The skin as a whole is composed of the soft dermis, the soft epidermis, and the 

 shell or cuticle which the latter secretes. The epidermis is typically composed of a 

 single stratum of chitin-producing cells, and often rests upon a thin basement mem- 

 brane, which then forms a distinct boundary between the two layers and Uke the outer 

 shell is a cuticular product. The dermis is composed of connective tissue cells, which 

 are often attached to the basement membrane, blood vessels, nerve fibers, pigment 

 cells, and glands, which are apparently of epidermic origin. Wherever muscles are 

 attached to the shell, the epithelium is greatly modified or reduced (see ch. vi, p. 241). 

 The shell in sectional view shows four layers, namely, (i) a thin outermost stratum, 

 which is structureless, called the enamel layer; (2) an underlying and lamellated pigment 



