243 



SECOND GREAT DIVISION OF THE ANIMAL 

 KINGDOM. 



ANIMALIA M0LLUSCA.(1) 



The Mollusca have neither an articulated skeleton nor a verte- 

 bral canal. Their nervous system is not united in a spinal marrow, 

 but merely in a certain number of medullary masses dispersed in 

 different points of the body, the chief of which, termed the brain, is 

 situated transversely on the oesophagus, and envelopes it with a 

 nervous collar. Their organs of motion and of the sensations have 

 not the same uniformity as to number and position, as in the Verte- 

 brata, and the irregularity is still more striking in the viscera, par- 

 ticularly as respects the position of the heart and respiratory organs, 

 and even as regards the structure of the latter; for some of them 

 respire elastic air, and others salt or fresh water. Their external 

 organs, however, and those of locomotion, are generally arranged 

 symmetrically on the two sides of an axis. 



The circulation of the Mollusca is always double; that is, their 

 pulmonary circulation describes a separate and distinct circle. 



The blood of the Mollusca is white or bluish, and it appears to 

 contain a smaller proportionate quantity of fibrine than that of the 

 Vertebrata. There are reasons for believing that their veins fulfil 

 the functions of absorbent vessels. 



Their muscles are attached to various points of their skin, form- 

 ing tissues there which are more or less complex and dense. Their 



(1) Soft animals. 



