58 
Tanks No. 20 and 22. 
and the fin swinging to and fro like a pendulum. Their protusible 
tongue is armed with sharp hooks and with it they catch the lesser pe¬ 
lagic forms. The same might be said of Carinaria (Fig. 96), which 
attains a considerable size and has a delicate trans¬ 
parent shell. 
The Pteropoda are curious, because externally 
they differ in almost every point from the typical 
Snails. The head is only indicated by the mouth 
and the rudimentary tentacles. The body is often 
covered by a delicate shell. The most striking 
feature is a pair of large wing-like fins, which are 
attached to the head or neck and are used by the 
animals as wings; hence the Neapolitan name far- 
falle di mare (Sea-butterflies). The most common 
genus Hyalaea (Fig. 97) has a delicate horny shell 
of brownish colour, and large fins which are per¬ 
petually beating. It appears in swarms, but only lives a day in the 
Aquarium (Tank 20). 
The oysters , mussels etc. belong to the Bivalves (Lamellibranchia). 
the lowest group of the Mollusks. They are distinguished from the snails 
by their shell, which consists of two pieces which are hinged, and are 
brought together by means of one or two muscles, but open by an elastic 
external ligament when the muscles are relaxed. The absence of a 
head is a characteristic feature of this group. The protrusible "foot” 
(Fig. 105 on the left) serves as organ of locomotion. The body is covered 
on both sides by the leaf-like gills, and the latter by the two mantle- 
flaps which secrete the two pieces of the shell. This shell is like the 
binding of a book, the leaves of which are represented by the two mantle- 
flaps and four gill-plates. The cilia or hairs which cover the gills and 
the mantle, by their beating movement create a current which is constantly 
bringing fresh water from the surroundings to the gills, so that the latter 
are well aerated. At the same time this current serves to bring micro¬ 
scopic animals and other food material into the mouth of the animal (see 
p. 14). Those shells which burrow deep into the sand allow a pair of 
long tubes (siphons) to protrude a little, and through these take in and 
pass out a current of water (e. g. Solecurtus, Fig. 105). — The Bivalves 
are generally either fixed permanently like the oyster, or they burrow 
to some depth into the sand; a very few can swim about freely or can 
jump. (With the exception of Pecten the Bivalves will be found in 
Tank No. 22). 
The most important example is the Oyster, Ostrea edulis (Fig. 98). 
Everybody knows its unpretentious shell, which is usually fixed to a 
rock by the thicker half. In their youth the Oysters swim about freely 
in the sea, but they soon settle down and secrete a substance which glues 
the shell to the rock. The "foot”, which in most Bivalves is the chief organ 
of locomotion and assumes considerable dimensions,, becomes quite rudi¬ 
mentary in the Oyster, where it is no longer used. Each Oyster is both 
Fig. 97. Hyalaea 
trident at a. 
