ORGANS OF SUPPORT 



FOURTH SECTION. 



Organs of Support in the Cyclo-Gangliated, or Molluscous 



Classes. 



THE external skeletons of the molluscous animals are 

 consolidated by the carbonate of lime, without the phos- 

 phate which is common in the other great divisions of 

 the animal kingdom. This earthy matter is secreted from 

 the skin in successive layers mixed Avith a glutinous co- 

 agulable animal matter, which gives firmness and tenacity 

 to the whole mass, and the skeletons are not exuviable, 

 as in the other articulated classes. From the low condi- 

 tion of all the organs of relation in the molluscous ani- 

 mals, they are less able to perceive, or avert, or escape 

 from danger than the free and active articulata, and they 

 accumulate and preserve ah 1 the successive layers of their 

 rocky covering permanently in contact with their surface. 

 The sheUs of these animals are remarkable for their want 

 of symmetry on the two sides of the body, the want of 

 unity in their plan of formation, and their inconstancy in 

 animals of similar structure. 



XIV. Tunicata. The tunicated animals have no external 

 shell nor internal solid parts, but are covered with a tough 

 elastic homogeneous tunic, in form of an enveloping sac, 

 with a respiratory and an anal orifice. This exterior sac 

 is the analogue of the valves of conchifera, and has the 

 muscular fibres of the lining mantle inserted into its 

 inner surface, as in the shells of bivalvia. It presents 

 every variety of colour and consistence in the different 

 species, and has often a coating of extraneous particles 

 of shells or gravel adhering to its outer surface. Some- 

 times it extends at its lower part into numerous short 

 processes, or into a long peduncle to attach the body to 

 rock or other hard substances. This exterior cartilaginous 



