48 
PNEUMONOBRANCHIATA. 
veral species allied to the above. Eossmasler 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 bagJike 
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 live 
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- 
