CHAP. If.] FROM PORTSMOUTH TO TENERIFFE. ~ 198 
shells are, with few exceptions, univalve and spiral, often thin 
and delicate ; sometimes, as in the genera Strombus, Pusus, Co- 
nus, and many others, thick and massive—weighty accumula- 
tions of carbonate of lime secreted from the sea-water. They 
have a distinct head, bearing organs of sense; but the character 
which most distinguishes them from their nearer allies is their 
mode of locomotion, which is by means of a long muscular plate 
secreting a viscid mucus running along beneath the body of the 
animal, and by alternate extension and contraction enabling it 
to creep over a solid surface. Most of these animals live on the 
bottom of the sea, as their organization demands. One or two 
only of the shell-making genera are pelagic, and the only im- 
portant one of these is the genus J/anthina, which inhabits a 
spiral shell, like a snail-shell, of a most lovely blue. Janthina 
floats by spreading out its “ foot ” on the surface, but it is more 
usually found attached to the different kinds of “ Portuguese 
men-of-war,” Velella, Physalia, and Porpita, or in the Mid-At- 
lantic, in the wandering isiands of gulf-weed. At certain sea- 
sons a peculiar kind of membranous float or raft is secreted 
from the animal, like a crescentic piece of honey-comb with the 
cells filled with air. The egg-sacs, which are not unlike those 
of the common whelk, are attached beneath the float ; and when 
the float is complete, and the egg-sacs full, the creature disen- 
gages it, and leaves the eggs to be hatched as it drifts about on 
the surface in the warmth and sunlight. 
The shells of Janthina are common in the globigerina 
ooze. They are not unfrequently cast up on the shore on the 
west coasts of Ireland and Scotland, and even on the Shetlands 
and the Faeroe Islands. They are not, however, inhabitants of 
our Northern Seas. They are drifted along and scattered about 
by our beneficent ameliorator, the Gulf-stream. 
The Herrropopa are very close to the GAsTERopopa, and in 
most modern works on zoology they are associated with them 
as a subclass. They are entirely pelagic, and as it is only un- 
