12 



a regularly formed brain ; and here it is obvious to those who un- 

 derstand the difference between the growth of shell and bone that 

 the former would be unfit for the purpose, since there is no provi- 

 sion made for the enlargement of the original cavity. 



Radiata, Cuvier ; Cyclo-neura, Grant. — In this acrite or 

 lowest division of the animal kingdom the skeleton generally holds 

 an internal situation, and is composed either of one large mass or 

 several smaller pieces symmetrically disposed, composed of silicious 

 or calcareous spicula. In many of the polygastrics the organ of 

 support consists in a condensation of the common integument en- 

 veloping the body — occasionally in the form. of an elastic vagini- 

 form sheath into which the animal can retreat on the approach of 

 danger, (fee, as seen in the vaginicola innata. Among the pori- 

 phera, skeletons are met with of a horny, silicious, or calcareous 

 structure, variously modified. In none does nature seem to have 

 amused herself more in the construction of skeletons than among the 

 zoophytes. Here they are met with internal or external, soft, horny, 

 or calcareous ; branched, globular, or filiform ; free or fixed. The 

 astenas and others of the echinodermata present us with skeletons 

 in the form of external crusts or shells, disposed after the manner 

 of plates, and composed of carbonate, with a trace of phosphate 

 of lime. 



Articidata, Cuvier; Diplo-nerose Animals, Grant. — The bo- 

 dies of these animals are generally long, cylindrical, and divided 

 transversely into segments. Their skeletons are generally thin, 

 light, and situate externally, chiefly composed of phosphate of lime, 

 though occasionally of carbonate, as in the cirrhopoda and Crusta- 

 cea. The entozoa owe their peculiar stiffness and rigidity to the 

 tough, strong, and transparent covering enveloping their entire 

 body. In some of the inferior orders of them, as in the acanthoce- 

 phalous species, their retractile proboscis and part of their bodies 

 are set with dense, sharp spines, which enable them to move with 

 freedom and precision through the fleshy media in contact with 

 them. The rotifera are closely allied to the entozoa in their ex- 

 terior coverings. In the former, however, this texture possesses a 

 greater degree of firmness, from the attachment of numerous mus- 

 cles to it. There are no earthy deposits in any part of the body of 

 these animals. The cirrhopoda, like the mollusca, are usually 

 enclosed in shells, dense, thin, laminated, and composed of carbo- 

 nate of lime. These testaceous coverings are best developed in the 

 balani. and least in the anatiferse. The reverse order of develop- 

 ment obtains with respect to their extremities. 



The amielida, or red-blooded worms, lead us a step higher in 

 the development of skeleton; for, although the halithea, the leech, 

 the nais, &c, possess a flexible membraneous covering, many others, 

 as the serpulas, are shielded by adventitious, solid, calcareous tubes. 

 The common earth-worm is provided with four pairs of sharp 

 spines, or setas, for the purpose of progression. The skeleton of 

 insects is for the greater part composed of a thin, epidermic layer, 



