404 
THE OCEAN WOULD. 
a very small shell, as in Testaeella, is very exactly indicated hy 
Nature. Lima , or rufus, spoken of above, presents, under the posterior 
part of the cuirass, calcareous, unequal, isolated granulations, which are, 
so to speak, the elements, as yet internal, of a shell which is on the 
point of being built. Other species in the same genus present under 
the cuirass a little rough, imperfect scale, which seems to be produced 
by a great number of these calcareous granulations, which show them' 
selves in an isolated state in Limax rufus. 
Ll'MNEANS. 
The Limneans, Aquatic Pulmonary Gasterojoods, as we have already 
said, are the animals belonging to this group that have to come to the 
surface of the water to breathe, as the dolphins and phocas among the 
Mammifera do. The Limnea, Planorbis, and Physa are the prim 
cipal members of this group. 
The Limneans live in great numbers in the stagnant waters of all 
countries, particularly of temperate climates. They cannot remain 
long under water, being compelled frequently to rise to the surfa° e 
in order to breathe atmospheric air. They are even observed, by 
a mechanism not very well understood, to turn themselves upsid 0 
down, in such a manner as to present themselves feet uppermost 
and to move slowly along in this position, creeping, as it wer e > 
through the water. It is difficult to comprehend how the movabl 0 
liquid bed upon which the animal operates can offer resistant 
enough to permit of its creeping as if it were on a solid resisting 
body ; it seems to produce the movement with the assistance 
of its foot, which is broad and thick, and shorter than the 
shell. 
The Limneans have a large flat head, from each side of which issues 
a triangular contractile tentacle, carrying at its base and on the inner 
side an extremely small dot, or eye. The most considerable part of 
the body, comprehending the visceral mass, is spiral, and is contained in 
a thin diaphanous shell (Fig. 200), the turns in the spiral of which 
are generally elongated, the last turn being larger than all the others- 
The interior of this is occupied by the respiratory cavity, which com' 
municates outwardly hy an opening analogous to that which exists m 
