THE GASTEROPODA AND PTEROPODA. 433 



prehensile processes, and its antero-lateral portions do not 

 extend beyond the sides of the head, and unite in front of the 

 mouth. 



In the other division (the Cephalopoda)^ the margins of 

 the foot are produced into prehensile processes or arms, and 

 the antero-lateral regions of the foot extend over, and unite 

 in front of, the mouth, in such a manner that the latter is 

 placed in the centre of the discoidal foot.^ 



In the former division — that is, in all Pteropoda — in all 

 those Gasteropoda which breathe the air dissolved in water 

 (Srayichiogasteropoda), and in some of those which breathe 

 air directly [Palmogasteropoda)^ the embryo is, as in the 

 Scaphopoda and Polyplacophora, a veliger ; or, at any rate, 

 it has ciliated bands which subserve locomotion. But in the 

 Cephalopoda no such velum is formed, and the animal ac- 

 quires the general characters of the adult before leaving the 



A shell-gland is often, if not always, present in the em- 

 bryo of the higher Odontophora ; and, in all Pteropods and 

 Branchiogasteropods, the mantle secretes a cuticular shell, 

 which, however, may exist only during the larval condition. 



If the arrangement of the alimentary canal in a Cephalo- 

 pod, or a Pteropod, be compared with that which obtains in 

 such a Branchiogasteropod as Atlanta^ it will be observed 

 that, in the former, the oesophagus enters the outgrowth of 

 the haemal region of the body which constitutes the visceral 

 sac, to reach the stomach ; and that the intestine passes, at 

 an acute angle with the anterior portion of the alimentary 

 canal, along the posterior face of the visceral sac, to end in 

 the pallial chamber, which is situated on the posterior face of 

 the body. The pedal ganglia consequently lie between lines 

 traversing the anterior and the posterior divisions of the ali- 

 mentary canal respectively ; and hence the alimentary canal 

 has a neural flexure^ or is bent toward the neural face of the 

 body. 



In Atlanta^ on the other hand, the intestine, when it leaves 

 the stomach, passes along the anterior face of the visceral 

 sac, to reach the pallial cavity, which is situated on the an- 

 terior face of the body. Hence lines traversing the two di- 

 visions of the alimentary canal would inclose not the pedal 



* See^ for a valuable discussion of the horaolosries of the arms and the funnel 

 of the Cejahalopoda^ in which the view here taken is ably, though I do not 

 think satisfactorily, controverted, Grenacher, "Zur Entwickelunjjsgeschichte 

 der Cephalopoden." {Zeitschriftfur wiss. Zoologie, 1874.) 



19 



