I.] INTRODUCTION. 



XV 



organization of Teredo and Chiton. All these divisions, 

 however, are clearly artificial. The Cephalic or Cephalo- 

 phorous Mollusks have a distinct head, and usually ten- 

 tacles or horns, with eyes at their tips or base, and a 

 foot or muscular disk for crawling or floating. A few of 

 them have arms for swimming. Some inhabit the land, 

 and others the water. The Snail, Whelk, and Cuttle- 

 fish are instances of this kind of Mollusca. 



Acephala or Bivalves. Although the bivalve Mollusca 

 have no heads as a distinct part of their bodies, they are 

 not deficient in those organs of outward sense which cha- 

 racterize other Mollusca, and their brain is as largely 

 developed. Many of them have numerous eyes for 

 seeing, otolites or ear-stones for hearing, filaments for 

 touching, lips for tasting, a mouth for taking in their 

 food, and (according to some physiologists) also an organ 

 of smell. The fry of the Oyster, Terebratula, and other 

 bivalves, which, in their adult state, are permanently 

 attached to other substances, swim about freely, and are 

 provided with distinct eyes, which afterwards disappear. 

 All the Acephala are aquatic, and respire by means of 

 gills, like fishes. The Oyster, Mussel, and Cockle are 

 familiar examples of this kind of Mollusca. 



Tunicata. The Tunicata or Ascidians can scarcely 

 be said to belong to the Mollusca, from which they differ, 

 according to Milne-Edwards, in many essential par- 

 ticulars, especially as regards their circulation and re- 

 production. They appear to have a considerable affinity 

 to the Polyzoa (or what are generally called " Polypes"), 

 and may with them constitute the great and separate 

 class of Molluscoidea. Each division of the animal 

 kingdom has so many points of resemblance to others, 

 and the network of organization is so closely inter- 

 woven, that it would be very difficult to define any one 



