22 ORGANS OF SUPPORT, 



In the holothuria, which are animals of this kind, there 

 are only five pairs of small calcareous pieces disposed around 

 the mouth like the dental apparatus of the echini. These 

 pieces give support to the ramified tentacula, and afford a 

 firm attachment to the strong longitudinal muscular bands 

 which encompass the body. 



THIRD SECTION. 



On the Organs of Support in the Diplo-neurose, or 

 Articulated Classes. 



The animals of this great division have the trunk of the 

 body for the most part long and cylindrical, divided trans- 

 versely into segments, and provided with numerous pairs 

 of organs of motion symmetrically disposed along the sides. 

 From their activity, their skeleton is generally in form of a 

 light, thin, exterior, enveloping, condensed integument, to 

 the inner surface of which the muscles are attached through 

 the medium of the cutis, and which is periodically cast 

 and renewed, to allow of the growth and increase of the en- 

 closed soft parts. The articulated appearance of the skele- 

 tons in these animals is generally proportioned to the density 

 of its texture. Where the whole skeleton is soft and flexi- 

 ble, there are few or no traces of articulations, and where its 

 texture is dense and unyielding, the articulations are most 

 distinct and complete ; hence in the helmithoid classes, and 

 in the young state of the entomoid animals, the articulated 

 appearance of the skeleton is less distinct than in the adult 

 forms of the articulated animals with articulated members. 

 The tubular form of the hard enveloping parts of the arti- 

 culated animals, and their unorganized nature require them 

 to be exuviable, and as they are thus the result of a single 

 effort of formation, they are always thin and light coverings. 

 They are not reinforced by successive deposits added to the 

 surface, through the whole of life, as in the molluscous 

 classes, and consequently we find but little of that softer 

 material, the carbonate of lime, employed in their consolida- 

 tion. The more dense material of the phosphate of lime 



