Tanks No. 10, 20 and 22. 
47 
Most genera of the Amphipoda, the next group, of which the reader 
perhaps knows the common Freshwater Shrimp (Gammarus pulex), are 
marine forms. Phronima, the Hermit-screw (Fig. 75), is especially inter¬ 
esting. It is a perfectly transparent pelagic form, and curiously enough 
makes use of the young Pyrosoma (see p. 64) as a dwelling, eating out 
its centre so as to form a small barrel. It fastens itself to this house by 
means of its front legs and protrudes the hind end of its body, the legs 
of which perform rapid strokes, which propel the animal together with 
its house through the water. This invertebrate 
Diogenes uses its transparent tub as nursery 
too; not only fastening the eggs to its wall, but 
keeping the young there for some time after 
they are hatched. It is caught on the surface 
of the sea, together with jellyfish and other ''pe¬ 
lagic forms” especially in the months of winter 
and spring, and will be found occasionally in 
tank No. 20. 
The lowest division of the Crustacea shown Fig. 75. Phronima sedentaria 
in the tanks is that of the Cirripedes ("Tendril- barrel. Tank 20. 
feet”), popularly termed Barnacles, which are 
externally so unlike shrimps or crabs, that they have only in recent times 
been properly understood. Even Cuvier looked upon Balanus, the^Acorn- 
barnacle (Fig. 76), and Lepas, the Goose-barnacle (Fig. 77), as mollusks; 
Fig. 76. Palanus pejforatus, on a 
rock; i/ 2 nat. size. Tank 22. 
Fig. 77. Lepas anatifera hanging to a 
floating piece of pumice-stone, ^2 nat. size. 
Tank 10 or 22. 
and it was not till much later that their early stages, and their anatomy, 
revealed the fact that they belonged to the Crustacea. 
The general public will therefore also experience some difficulty in 
accustoming its mind to the fact that these animals are undoubtedly 
relations of the well-known forms of Crustacea. This may be more in¬ 
telligible when it is told why we suppose that the curious form of the 
animal, reminding one of the shell of some fixed mollusk, is due to a far- 
reaching degeneration. In their early youth, these animals are very small, 
active and free-swimming, with a pear-shaped body and three pairs of 
swimming-legs. This larval stage is common to all the lower forms of 
