4 BRITISH MOLLUSKS. 



respiration, possess an apparatus which gives them the faculty of 

 breathing air in cases of emergency, such, for example, as when the 

 ponds in which they are dwelling become dried up. 



Dr. Gray is of opinion that the shell and its operculum are homo- 

 logous to the two valves of a bivalve, and that the normal form of a 

 mollusk is to be protected by two valves or shells. This is an inge- 

 nious theory, in which few participate to the extent of the author. 

 The operculum affords fair subsidiary characters for the distinction 

 cf groups. The foreign Cyclostomacea, numbering eight hundred 

 species, are subdivided on this method into as many as forty genera. 

 The arrangement is almost universally adopted, but it embraces, 

 in some instances, a rather anomalous association of forms. 



The Inoperculated Cephals are subdivided into two Orders : — 



1. Pulmonifera. Respiratory organ a vascular sac for the respi- 



ration of ah\ 



2. Pulmobranchiata. Respiratory organ a vascular sac for the 



respiration of air, with the addition of branchial lamellse 

 (gills), for the respiration also of water. 



Order I. PULMONIFEBA— air-breathing. 



Respiratory organ a vascular sac for the respiration of air. 



One-half of our entire series of Land and Freshwater mollusks 

 belong to this Order. They include the Slugs, Limacinea, the 

 Snails, properly so called, Colimacea, all of which are terrestrial, 

 and the Auriculacea, which Live near the sea, within reach of the 

 spray, or on the banks of rivers or of estuaries. The pulmonary 

 organ or lung is a sac over the surface of which the blood flows in 

 minute vessels for the purpose of being aerated. 



Although breathing air as their natural element, the Pulmonifera 

 love damp and moisture, and sustain life with comparatively Uttle in- 

 convenience for a long time under water. A slug may not uncom- 

 monly be seen attached to the inner side of a waterbutt, in which it 

 has become immersed by the rising of the water, without evincing 

 any disposition to move ; and a snail, on being dropped in a glass of 

 water, will put forth its tentacles with Httle less than its accustomed 

 ease, and crawl up leisurely to the brim, with an air-bubble at its 

 respiratory orifice, Like that of the water-beetle. It may, however, 

 be observed that the slug, when submerged, secretes a more copious 

 discharge of mucus, and forms for itself a kind of viscid envelope. 



