CHAP. IV.] THE NERVOUS SYSTEM OF MOLLUSKS. 77 



stitute, in the northern seas, the principal article of food 

 of the great whales." 



Some of the least highly organized members of this 

 class, such as the Hyalleidse, possess a bivalve shell, and 

 no distinct head ; but in other Pteropods devoid of a shell, 

 we meet with a higher organization. Thus in Clio there 

 is a distinct head bearing sensory appendages, in the form 

 of two tentacula and two eyes, and containing ' a brain ' 

 within. The brain is represented by two connected ganglia 

 above the oesophagus, which are in relation, by means of 

 ingoing nerves, with the above mentioned sensory organs. 

 In connection with another commissure uniting these two 

 cerebral ganglia and which passes under the first part of the 

 alimentary canal, are two .'pedal' and two 'branchial' ganglia 

 pretty close together. These two pairs of ganglia exist 

 separately in Clio and its allies, though they are combined 

 into one quadrate mass in Hyalea. In Clio two ' auditory 

 saccules ' are in connection with the anterior sub-cesopha- 

 geal ganglia that is, with the pair which corresponds 

 with the 'pedal' ganglia of the common bivalve Mollusks. 



Gasteropods constitute a class of organisms which, in 

 point of numbers, can only be compared with the still 

 more numerously represented class of Insects. Their 

 name is derived from the fact that they crawl by means of 

 a large muscular expansion or ' foot ' stretched out beneath 

 the viscera. The locomotion of members of this class 

 may be said to be, in the main, dependent upon their own 

 individual efforts, so that, in this respect, they differ 

 widely from Pteropods, whose movements from place to 

 place are brought about chiefly by winds driving them 

 along the surface of the water on which they float. 



Some Gasteropods are terrestrial, air-breathing animals, 

 though by far the greater number are aquatic and breathe 



