OR OSSEOUS SYSTEM. 



SECOND SECTION. 



Organs of Support in the Radiated, or Cycto-neurose Classes. 



THE skeletons are as various as the forms of the animals 

 in this lowest division of the animal kingdom, and scarcely 

 indicate in their composition or structure a determinate plan 

 common to the whole. They are sometimes external, most 

 frequently internal, often composed of minute pieces sym- 

 metrically arranged, or of one solid mass, often of a thin 

 flexible diaphanous horny consistence, or composed of dense 

 silicious or calcareous spicula, or of masses of carbonate 

 with a little phosphorate of lime. The osseous parts in 

 these classes appear to be extravascular, and to grow by the 

 juxtaposition of new portions, and from the simplicity of the 

 general structure and functions of these animals, and the in- 

 ternal situation of their solid parts, they are not exuvi- 

 able. 



I. Polyyastrica. Many of the minute and soft polygastric 

 animalcules possess an exterior firm, elastic covering, 

 which protects the more delicate internal parts. This 

 covering sometimes consists only of a more condensed form 

 of the common integument, enveloping every part of the 

 body, in others it forms a distinct thin pellucid sheath into 

 which the animal can withdraw its soft parts for protection. 

 The exterior surface, even of the softest and most naked 

 animalcules, supports the organs of motion the minute vi- 

 bratile cilia by which they are carried to and fro, and consists 

 apparently of a thin film of the general cellular tissue of 

 their body, rendered more firm in its texture by the con- 

 tinued action of the surrounding element. This condensa- 

 tion of the exterior integument is the origin of most of the 

 skeletons of invertebrated animals, which have generally 

 the organs of support thrown over the surface of their body 

 to afford them at the same time protection. We have an 

 example of one of these loricated animalcules in the volvox 

 globator, (Fig. 1. A) so common in stagnant pools of fresh 

 water, and which often owe their green colour to the abun- 

 dance of this animalcule. This sperical transparent green 



B 2 



