Pteropoda. MOLLU SCA.- 
■ -Pteropoda. 
fins for progressive motion. They are found far at sea, 
in large herds, swimming about in a lively manner by 
undulatory or flapping movements of their membranous 
fins, which open and close like those of the butterfly 
while basking in the sun. They shun the lightsome 
day, and sink, as the sun rises, into the bottom of the 
deep, to attain that shaded gloom which suits them; 
but on the evening’s approach they gradually again 
ascend to the surface, and regain all their vivacity; 
so that if some will fancifully seek amongst them the 
molluscan analogues of insect-butterflies in their manner 
of natation, they will not fail to mark the correspond- 
ency between them and the moths in their crepuscular 
and nocturnal habits. The little Hyales {Cavolinidm) 
first appear about five in the afternoon. When the 
garish eye of day begins to grow dim, two or three 
species venture upwards to the field of their occupancy ; 
as evening advances several small species of Cleodores 
{Cleodora) rise in great number with other Hyales and 
Atlantes, but the larger kinds do not leave the abyss 
and mingle in the crowd until night lends them her 
friendly veil ; and some species, as the Hyalaea balan- 
tium {Balantium recurvwn) are even so fearful of the 
light’s malign influence, that they do not come to the 
357 
surface excepting when the night is very dark. After 
a few hours’ disport the lesser species begin to descend 
and disappear ; the larger follow at a little later hour, 
so that towards midnight only a few wandering indi- 
viduals can be taken. These may possibly remain even 
to the dawn, but the sun’s rise is the signal which recals 
them to their home. After this not a single Pteropod 
is to be seen, either at the surface or at any depth to 
which the eye can penetrate. Each species has its 
own time at which it rises up and goeth down, deter- 
mined not by the clock, as you will readily believe, but 
by the degree of obscuritj’ in the heavens, so that in an 
overshadowed day they rise earlier than in a cloudless 
one, and sink earlier also to repose.” 
The Pteropods are carnivorous animals, feeding on 
minute crustaceans, as the Entomostraca, &c., and 
small Medusee. They form great part of the food of 
the whale, in high latitudes, and are also devoured in 
great numbers by many sea-birds. 
The Pteropoda are divided into two groups or orders 
according as they are furnished or not with a shell. 
Those in which the body is inclosed within a thin 
shell, are called Thecosomata\ and those which are 
destitute of any such appendage, Gymnosomata. 
Order I. — THECOSOMATA (Shell-bearing Pteropods). 
The species of Pteropods belonging to this order are 
distinguished by having their body inclosed within a 
shell. 
In most of the species the body and shell are straight 
or globular, and there is no operculum ; hut in one 
family {Limacinida:), the animal and shell are spiral, 
and there is a spiral operculum present. 
(in the young) an acute apex, which, however, falls off 
in the adult, and is separated from the anterior cavity 
by a partition. The animal has an elongate, cylindrical 
body, and has two large lateral wings or fins in front, 
united below bj^ two small lobes. The genus Cuvieria 
{=Triptera) is the only one in the family, and contains 
three recent species and one fossil. 
Family— CAVOLINIDAl (= Hyalceidm). 
The Hyalaeas have a calcareous, symmetrical shell, 
which is sometimes straight, at others curved ; some- 
times globular, and at others needle-shaped. The 
animals have two united fins, without any posterior foot- 
like appendage between them. It is by means of these 
fins that the animals swim through the water ; and it 
has been observed that when they are touched, they 
retract them and fall to the bottom of the vessel in 
which they were detained as captives. 
The family consists of five distinct genera : — 
1. HyaLjEA (= Cavolina), with a globular trans- 
lucent shell. Eepresented on Plate 11, fig. 28 {Hyalma 
tridentala). 
2. Diacria, with an elongated triangular shell. 
3. Cleodora (— Clio), with an elongate, pyra- 
midal, hyaline, three-sided shell. 
4. Balantium, with a triangular, depressed, trans- 
versely-waved shell ; and 
5. Styliola (= Creseis), with a slender, elongate, 
conical shell of a subcylindrical form. 
F AMiLY — TRIPTERID Al ( Cuvieriidce). 
The Cuvieras have a glassy, transparent, cylindrical 
shell, with a transversely ovate, simple aperture, and 
Family— CYMBULIIDiE. 
The Slipper Shells have a cartilaginous sort of shell, 
shaped very like a slipper, pointed in form, and trun- 
cated posteriorly. The animal is globular or ovate, 
and has large, rounded fins connected ventrally by a 
small intermediate lobe. This family contains four 
peculiar-looking, pelagic genera, of which the genus 
Cymbidia is the type. Tlie shell which incloses the 
animal is of agelatino-cartilaginous nature, of an oblong 
figure, and shaped very like a slipper. It contains four 
species. “ During the day,” says Mr. Adams, “ these 
animals must occasionally descend a considerable depth, 
one having been brought up attached to a thermometer 
of a sounding line, during the voj'age of H.M. ship 
Samarang, from one hundred and fifty fathoms in the 
South Atlantic.” 
Family— LIMACINID^. 
The Limacinas have a small transparent, sinistral, 
spiral shell, with the mouth angularly produced on the 
columella side, and closed by a distinct, spiral, vitreous 
operculum. The animal has a .spiral, sinistral bodj', 
with fins attached to the sides of the mouth and united 
ventrally by a lobe which bears the operculum. 
