78 BULLETIN OF THE UNITED STATES FISH COMMISSION. 



At the time the shell is ready to be cast the tegumentary covering consists of 

 (1) the old shell; (2) the new shell; (3) an intermediate structureless membrane, 

 besides the chitinogenous epithelium, and (4) the dermis. The new carapace, according 

 to Vitzou, is composed of the enamel and pigment layers only. The calcified layer is 

 not formed until after the molt. 



The connective-tissue cells are now of large size and contain granules of glycogen. 

 Claude Bernard first demonstrated the presence of glycogen below the carapace in 

 Crustacea. Glycogen was extracted by Vitzou from the connective tissue, liver, 

 lymph, and ovaries, during the molting period. Besides acting as a pancreas, the 

 liver was found to be a great producer of glycogen during the molt. Glycogen is 

 thus an organic reserve, which furnishes material for the growth of the new shell and 

 tissues. According to Vitzou, Schmidt and Berthelot have shown that the chitin of 

 the Crustacea contains a principle belonging to the same group as cellulose and lignin. 

 This substance, under the influence of sulphuric acid, may be transformed into a body 

 analogous to glucose. Hence the remark of Claude Bernard, that " without forcing 

 the metaphor one may say that the Crustacea are enveloped in a carapace of wood." 

 (Lecons sur les phenomenes de la vie, 1879. t. 2, p. 113. — 197.) 



The enamel is plainly the first product of the secretions of the skin which goes into 

 the new shell, and when once laid down can not be competely removed except by a 

 molt. The enamel is often partially removed by friction, as is seen in the abrasions 

 on the shells of old lobsters or those about to molt. 



The surface of the shell, particularly that of the carapace, has a decided punctate 

 appearance, due to the hair pores. These mark the points where setse either pene- 

 trate the shell now or did so at an earlier stage of development. In the adult lobster 

 the seta? of the carapace have disappeared or are worn down except upon its margins 

 and in the orbital regions. In the fourth larva, however, the whole carapace is seen 

 to be studded with hairs (fig. 113, 115, plate 35). 



If the carapace — better one without pigment — is examined with a hand lens, the 

 surface is seen to have a beautiful though somewhat irregular mosaic appearance. 

 It is divided into polygonal areas which inclose the hair pores. These markings are 

 probably due, as Professor Patten has shown to be the case in Limulus, to shallow 

 depressions in the enamel, dependent upon a peculiar bendiug together or clustering 

 of the outer ends of the canaliculi. The hair pores open on the inner side of the 

 shell in small blister-like elevations. 



A very minute pore of another character is scattered among the hair pores on the 

 inner side of the shell. It has the appearance of a symmetrical crater with a minute 

 tube issuing from it. This is the duct and opening of the tegumental gland. The 

 distribution of these two kinds of pores would probably repay careful study. But 

 few parts of the carapace, such as the white tendon marks (see p. 135), are wholly 

 free from them. Toward the lateral margins of the carapace they become exceedingly 

 small and numerous. Some of the superficial pits in the dorsal region, on the other 

 hand, are very large. 



We thus see that the dense shell is a veritable strainer, being perforated by 

 hundreds of thousands of minute passages, which lead from the surface to the parts 

 below it, to the tegumental glands on the one hand, or to the sensory cells which lie 

 at the roots of the hairs, on the other. The bearing of these facts will be better appre- 

 ciated when we discuss hereafter the function of the tegumental glands themselves. 



