48 PNEUMONOBRANCHIATA. 



veral species allied to the above. Rossmasler reduces 

 them to three; but, from the specimens which I 

 have received under different names, I greatly doubt 

 if they are more than mere local varieties of our 

 species. Nilson found a small variety or species in 

 Sweden, on the shores of the Baltic, with Mytilus 

 edulis, Cardium, &c. Our species has been found in 

 similar situations in Loch Stennis, Orkney, by Mr. 

 Edward Forbes. 



ORDER V. PNEUMONOBRANCHIATA. 



THE respiratory organs consisting of a number of 

 pulmonary vessels spread over a closed bag-like 

 cavity on the back of the neck. The edge of the 

 mantle adherent to the back of the neck of the 

 animal, forming a closed pulmonary chamber, leaving 

 only a hole for the entrance and exit of the air, 

 which is closed by an external valve on the side of 

 the cavity. They breathe free air, and either Jive 

 constantly on the land or in the water, in which 

 latter case they come periodically to the surface to 

 respire. The larvae are shell-bearing, shaped like 

 the adult, and destitute of any cephalic fins. The 

 lingual membranes are furnished with numerous 

 longitudinal series of similar, sub-equal, teeth. 



The shell is rarely wanting. 



They are all destitute of any operculum, but close 

 the shell, during the torpidity of the animal (pro- 

 duced by either dryness or cold), with a lid or epi- 



