101 



Second Great Division of the Animal Kingdom, 



ANIMA LIA MOLLUSCA. 



Mollusca have no articulated skeleton or vertebral canal. 

 Their nervous system consists of a number of medullary 

 masses, which are named according to their location in dif- 

 ferent parts of the body. The principal one is called the 

 brain, and is situated transversely on the oesophagus, which 

 it envelopes with a nervous collar ; organs of motion, circu- 

 lation and respiration various. The circulation is always 

 double, being assisted by an aortic ventricle seated between 

 the veins of the lungs and the arteries of the body ; blood 

 white or bluish. 



Nearly all the Mollusca have a development of skin called 

 a mantle ; this is either membranous or fleshy as in the naked 

 Mollusca, or it has developed in its thickness a substance 

 more or less hard, arranged in layers : when this substance 

 becomes so much developed that the animal finds shelter 

 beneath it, it is called a shell, and the animal is called testa- 

 ceous. 



All modes of mastication and every variety of digestive 

 apparatus is to be found in this class. They most generally 

 have a large liver. 



Their muscles are attached to different parts of the skin, 

 so as to produce various inflections, contractions or prolon- 

 gations of the body, by means of which they creep or swim. 



Explanation of Fig. A, PL 29. The nervous system of an 

 Octopus: 1, the brain; 2, the nervous collar around the 



