STRUCTURE OF THE MOLLUSCA. 187 



animals when first produced have nearly the same form as 

 they are to retain, and are not subject to metamorphoses, 

 or changes like those undergone by insects. 



As the organs are modified in the difl^erent classes, it may 

 now be well to present the principal characters of these 

 groups. 



CLASS I.-CEPHALOPODA. 



The class of Cephalopodous MoUusca is composed of ani- 

 mals of very singular form, fig. 1. Their trunk or body, 

 generally rounded, is enclosed in the mantle, which has the 

 form of a bag, sometimes nearly spherical or oval, sometimes 

 elongated, fig. 1, ab ; their head, fig. 1, d, is large, and 

 crowned with fleshy arms or appendages, fig. \,fg; sub- 

 servient to locomotion, as well as prehension; so that when 

 they crawl, it is with the head below, and the body above. 

 They are divided into two families: the Dibranchial, and 

 the Tetrahranchial. 



The Dibranchial, or Ordinary Cephalopoca, have 

 the most complex organization of all the Mollusca. Their 

 head is round, and furnished with two large eyes, similar in 

 structure to those of the vertebrated animals. They have 

 also a small auditory apparatus. Their mouth is armed 

 with two strong horny jaws or mandibles, like a parrot's or 

 hawk's bill ; and around it are disposed in a circle, long, 

 tapering, fleshy bodies, fig. ^,fg, capable of being moved 

 in any direction, muscular and vigorous, and furnished with 

 cup-like bodies, by which they can be very firmly affixed to 

 objects on which they lay hold. The rest of their body is 

 contained in a kind of rounded bag, fig. \, a h, formed by 

 the mantle, or fleshy skin. 



These animals are all inhabitants of the sea, and respire 

 by means of branchiae placed on each side of the body, at 



