40 
Tanks 22 and 23. 
delicate auditory hairs and opening to the outside by a fine slit. The 
waves of sound set the hairs vibrating, and this motion is transmitted to 
the auditory nerve. Within the sack are the so-called auditory stones, 
chiefly grains of sand, which the shrimp puts into the ear-sack with its 
claws, and which would seem to intensify the vibrations of the hairs. 
But, as at each moult this sack together with its contents is cast off, 
Fig. 59. Penaeus caramote, V 2 nat. size. Tank 22. 
the animal has then to get a new supply of sand and in the meantime 
is probably somewhat hard of hearing. 
Two rare Shrimps, not always present in the Aquarium (tank No. 22), 
are Stenopus (Fig. 58), and 
Penaeus (Fig. 59). The latter 
is exceedingly palatable, and, 
as it has a very thin skin, 
would be very valuable as an 
article of food, if it were only 
a little more common. In the 
Bay of Gaeta it is largely caught 
under the name of "mazza- 
cuogna”. 
We now turn to a curious 
group intermediate between the 
long-tailed forms, the Lobsters 
and Shrimps, and the short¬ 
tailed Crabs. These are the 
Hermit-Crabs (Figs. 60—-62). 
A whelk - shell walking 
about on crab’s legs and carrying 
Fig. 60. Pagurus striatus in a whelk-shell and sea-anemones on its back 
bearing three Anemones, 1/2 nat. size. such is the impression made on 
Tank 23. an y one looking for the first time 
on a Hermit-crab (tank No. 23). 
This droll figure fascinates us at once and creates in us desire to 
know something more about it. The problem is simpler than it seems, 
and has the following explanation. The Hermit-crabs live in deserted 
whelk-shells. When they escape from the egg they are just like the young 
long-tailed prawns; but very soon their previously straight tail begins to 
twist itself spirally, and the time has come for the small animal, which 
is scarcely half an inch long, to look out for a shell. If it finds an empty 
