272 OPERCULATA. 



Order OPERCULATA. 



Teeth recurved, hooked, in seven rows, arranged in a 

 semi-lunar manner on a narrow lingual hand. Mouth 

 proboscidiform, not provided with horny jaws. Respiratory 

 organ reticulate, in the form of a sacciform cavity on the 

 hack of the neck ; edge of the mantle free from the nape, 

 leaving the respiratory cavity open. Sexes distinct. Oper- 

 culigerous. Animal oviparous, for the most part terrestrial, 

 and respiring free air. 



Dr. Leach appears to have been the first to have pointed 

 out the important differences in organisation which distin- 

 guish the operculate division of the Pulmonifera. Under 

 the name of Antrobranchia he has defined them as being 

 unisexual, and as having reticulated gills. Dr. Gray has 

 more recently shown, that in the armature of their lingual 

 membrane they closely approach the Pectinibranchiate 

 Gasteropods, the teeth greatly resembling those of the 

 Naticid(e. The embryonic state of these animals is not 

 well known, but from the circumstance of the eggs being 

 deposited out of the water, it is most probable that the 

 young resemble in form their parent, instead of being pro- 

 vided, as in the Branchiferous Gasteropods, with deciduous 

 cephalic fins, which, in their case, would be superfluous 

 appendages. The great majority of the operculigerous 

 Pulmonifers respire free air by means of their sacciform, 

 lung-like breathing organ. Their tentacles are simply con- 

 tractile, and not retractile by inversion as in the Helicida, 

 and their eyes are usually sessile on the head near the 

 bases of the tentacles, instead of being elevated on pedun- 

 cles as in that family. Some of them live on the ground 



