SHELLS OF CRUSTACEA. 171 



584. Among the Articulated classes, we still find that the skeleton is 

 altogether external, and belongs therefore to the cutaneous system ; but 

 it is formed upon a very different plan from the shells of the Mollusca, 

 being closely fitted to the body, and enveloping every part of it ; conse- 

 quently it must increase in capacity, with the advancing growth of the 

 contained structures. Moreover it is destined not merely to afford sup- 

 port and protection to these, but to serve for the attachment of the mus- 

 cles by which the body and limbs are moved : and the hard envelopes 

 of the latter serve, like the bones of the Vertebrata, as levers by which 

 the motor powers of the muscles are more advantageously employed. 

 Again, the hard envelopes of the body and limbs are not formed of dis- 

 tinct plates, like those of the Echinus-shell ; but are only divided by 

 sutures at the joints, for the purpose of permitting the requisite freedom 

 of motion. It might have been thought that here, if anywhere, a process 

 of interstitial growth would have existed, to adapt the capacity of the 

 envelopes to the dimensions of the contained parts, as the latter increase 

 with the growth of the animal ; but, true to the general principle, that 

 epidermic structures are not only extra-vascular, but that they undergo 

 no change when they are once fully formed, we find that the hard enve- 

 lopes of Articulated animals are thrown off, or exuviated, when the con- 

 tained parts require an increase of room ; and that a new covering is 

 formed from their surface, adapted to their enlarged dimensions. 



285. This is well known to occur at certain intervals in Crabs, Lob- 

 sters, and other Crustacea ; which thus exuviate not merely the outer 

 shell, with the continuation of the epidermis over the eyes, but also its ' 

 internal reflection, which forms the lining of the oesophagus and stomach, 

 and the tendinous plates by which the muscles are attached to the lining 

 of the shell. A similar moulting may be observed to occur in some of 

 the minute Entomostracous Crustacea of our pools, every two or three 

 days, even after the animals seem to be full grown. During the early 

 growth of Insects, Spiders, Centipedes, &c., a similar moult is frequently 

 repeated at short intervals ; but after these animals have attained their 

 full growth, which is the case with Insects at their last change, no fur- 

 ther moulting takes place, the necessity for it having ceased. This 

 moulting is precisely analogous to the exfoliation and new formation of 

 the Epidermis, in Man and most other Vertebrata ; differing from it 

 only in this, that the latter is constantly taking place to a small extent, 

 whilst the former is completely effected at certain intervals, and then 

 ceases. We have examples of a periodical complete m/oult in Vertebrata, 

 however, among Serpents and Frogs. 



286. The structure of the hard envelopes of Articulated animals cor- 

 responds with that of the Epidermis and its appendages in Man. The 

 firm casings of Beetles, for example, are formed of layers of epidermic 

 cells, united together, and having their cavities filled by a horny secre- 

 tion. The densest structure is found in the calcareous shells of the 

 Crustacea; which consists of a substance precisely analogous to the 

 Dentine of Teeth ( 311); covered on the exterior with a layer of 

 pigment-cells. The calcareous matter consists chiefly of carbonate 



f lime ; but traces of the phosphate are also found. The animal basis 

 has a firm consistent structure, resembling that of teeth. A thin ver- 



