54 
Tanks No. 3 and 22. 
this acid fluid in self-defence; it is still a puzzle how these strong cor¬ 
rosive acids are produced by the animal and how they can be stored up. 
An interesting animal is the Worm-shell, Vermetus (Fig. 88, 
Fig. 87. Doliwm galea, 73 nat. size. Tank 3. 
Tank 22), from the fact that it is not able to move about freely like the 
other snails, but is fixed to one spot. At first sight its shells are just 
like the calcareous tubes of Protula (see p. 34). But on closer inspection 
may be recognized the head of the 
Fig. 88. Vermetus gigas, two living 
specimens and one empty shell, 
72 nat. size. Tank 22. 
snail with its two short tentacles, 
very different from the brightly 
coloured gills of the worms. These 
animals feed on the small Crustacea 
and worms in their neighbourhood, 
and when alarmed, they withdraw 
themselves into their tube. To 
the inner wall of the latter they 
also fix their eggs; from these 
free-swimming larvae are hatched, 
which afterwards fix themselves to 
some rock. 
Allied to the above mentioned 
forms is a group of Mollusks 
which differ in having their gills 
attached behind their heart instead 
of in front. They are devoid of 
a shell or only possess a very 
small one which is hidden by 
the mantle, as in the land-slugs. 
This group includes : 
The Sea-hare, Aplysia (Fig. 89, Tank 3), a fairly large dark-brown 
animal with two pairs of tentacles, the posterior pair of which are carried 
erect and are something like the ears of a hare. The mantle is continued 
