20 ZOOLOGY. 



composed of carbonate of lime, and sometimes consists of 

 a single piece or " valve," sometimes of two valves 

 united together by a hinge. 



The digestive tube is always completely separated 

 from the general cavity of the body, and is provided with 

 an anal opening. 



The higher groups possess an organ situated at the 

 entrance to the alimentary canal, which is commonly called 

 the " tongue ;" but which, on account of the minute 

 flinty teeth with which it is furnished, Professor Huxley 

 has termed an odontopkore (Gr. odous, a tooth ; pkero, I 

 carry), or tooth-bearer. It works backwards and for- 

 wards on a cartilaginous cushion like a chain saw. It is 

 by means of this odontophore that the slugs and snails 

 make such havoc in our gardens. 



The blood in this group is colourless. There is a dis- 

 tinct heart, with two cavities, which occupies the dorsal 

 position. The blood passes from the gills, or other 

 breathing organ, to the auricle, thence to the ventricle, 

 which propels it through the body. 



The nervous system is very distinctive. It consists of 

 three pairs of ganglia united by nervous cords. One of 

 these pairs, situated in front above the oesophagus, is 

 termed the cerebral ganglia; another, which sends branches 

 to the foot, is termed the pedal ganglia; and the third. 



FIG. 3. DIAGRAM OF A MOLLUSC, 



a, digestive tube ; I, heart ; c, foot ; d y cerebral ganglia ; e, pedal ganglia ; 

 /, parie to-splanchnic ganglia. 



which communicates with the walls of the body, as well as 

 with the digestive and reproductive organs, is termed the 



