CHAP. I. ] FROM PORTSMOUTH TO TENERIFFE. 125 
they are very soft, formed mainly of a “connective tissue,” 
with little in it but sea-water. In this way their bulk is greatly 
increased without materially adding to their weight, and they 
weigh little more than an equal bulk of sea-water, and require 
little exertion to float or swim. 
One curious result of this transparency is that we can see 
through the outer wall, in the most wonderful detail, all the in- 
ternal arrangements—the nervous centres, with the complicated 
organs of sense; the heart, with its pulsating chambers, and the 
blood following its course through the system and through the 
gills; the alimentary canal, and all its accessory glands. The 
Hereropopa are probably the most highly organized group in 
which such transparency exists. 
The shells of Carinaria are rare in the globigerina ooze ; 
but two small spiral shells belonging to animals of the same 
subclass — Atlanta peronit and Oxygyrus kéraudrenii—are 
sometimes in such numbers as to have a sensible effect in add- 
ing to the formation. Although the Heteropod shells of the 
present day are insignificant in size, they played a much more 
important 7dde in early times; for there seems little doubt that 
the great shells of the genera Huwomphalus and Bellerophon, 
which sometimes go far to make up whole beds of limestone 
of the Silurian and Carboniferous periods, are to be referred 
to this group. 
The Preropopa are farther removed than the Hrrrropopa 
are from the typical Gasrrropopa, and are much simpler in 
their structure. The head is not so markedly separated from 
the body, and the organs of sense are rudimentary. The body 
is conical and sometimes spiral, and is very usually contained in 
a delicate shell, sometimes spiral in form, more frequently con- 
ical or tubular; or like an ornamental flower- glass, or like a 
watch-pocket. The foot is modified into two wing-like ap- 
pendages, one on either side of the mouth. These are free 
quently brightly colored when the animal is living, and differ- 
I.—9 
