THE HETEROPODA. 439 



a conical projection of the haemal surface, and the numerous 

 lamellar, or filamentous, respiratory organs, are lodged be- 

 tween the free edges of the mantle and the sides of the body. 

 In the other chlamydate Prosobranchiata, except the Cyclos- 

 tomata, there are two plumose gills lodged in a pallial chamber 

 situated on the anterior face of the visceral mass, which is 

 usually large and spirally coiled. Sometimes, as in the di 

 vision of the Aspidobranchia, the two branchiae are equal, or 

 nearly equal, in size. Sometimes one is so much smaller than 

 the other as to be nearly abortive ( Ctenobranchia). Arnpul- 

 laria has a pulmonary cavity as well as gills. On the other 

 hand, the Cyclostomata have no branchiae, but breathe air 

 by means of the parietes of the pallial chamber, whence they 

 are ordinarily reckoned among the Pulmonata, which they 

 resemble in their terrestrial habits. In many Prosobranchiata, 

 the wall of the branchial chamber is produced into a muscular 

 spout-like prolongation, termed the siphon, which serves to 

 direct the branchial current. The presence of this siphon is 

 usually accompanied by a notch or grooved process of the 

 shell, and by carnivorous habits. 



In the Hetcropoda, there is a gradual reduction of the 

 mantle, from Atlanta, in which the mantle and shell have the 

 ordinary proportions, and the departure from the ordinary 

 Gasteropod type is but little greater than that observed in 

 Strombus and Pteroceras, through Carinaria, in which the 

 mantle is much reduced, and the shell is a mere conical cap, 

 to Flrola, in which the mantle and shell are wanting in the 

 adult, and which, therefore, corresponds with the achlamydate 

 Pteropoda and Opisthobranchiata. 



In many genera of the Ctenobranchia, and especially 

 among the carnivorous forms, the mouth is situated at the 

 end of a long proboscis, which contains the odontophore, and 

 a great part of the long oesophagus. This proboscis is pro- 

 truded and retracted by special muscles. 1 



The eggs are often laid in capsules secreted by the walls 

 of the oviduct. In Neritina, Purpura, and Bucc'inum, each 

 capsule contains a considerable number of ova, but of these 

 only a few (one in Neritina) become embryos, and devour 

 the rest. 2 



1 See the description of the proboscis of the Whelk in Cuvier's " Memoires 

 sur les Mollusques." 



2 Koren and Daniellssen, "Recherches sur le developpement des Pectini- 

 branches" ("Fauna littoralis Norvegiae," ii.. 1856), and Carpenter, "On the 

 Development of the Embryo of Purpura lapUlus" ("Trans. Micr. Society," 

 1854, and " Annals of Nat. Hist., 7 ' 1857). Claparede, " Anatomic und Entwicke- 

 lungsgeschichtc der Neritina fluviatilis." (" Archiv fur Anatomie," 1857. ) 



