Crabs, Lobsters, Shrimps. Barnarl- 



Fig. 167. Proninui sedeu- 

 taria in its barrel. 



hatched. A curious fact worth noticiog is that all females have in their 

 youth be"n males ; the new-born animals are not divided into the two 

 sexes but , figuring as males in their early life , when they have grown 

 older they take upon themselves the duty of producing eggs. 



Most genera of the Amphipoda, the next group, of which the reader 

 perhaps knows the common Freshwater Shrimp (Gammarus pulex) , live 

 in the sea. Phronima, the Hermit-screw (Fig. 167) , is especially inte- 

 resting. It is a perfectly transparent pelagic 

 animal, and curiously enough makes use of 

 the young Pyrosoma (see p. 85) as a dwel- 

 ling , 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, keeping the 

 young there for some time after they are 



hatched. It is caught on the surface of the sea, together with jelly-fish 

 and other pelagic animals especially in the mouths of winter and 

 spring , and will be found occasionally in tank Nr. 20. 



The lowest division of the Crustacea shown in the tanks is that of 

 the Girripeda ( Tendrill-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-bar- 

 nacle (Fig. 168), and Lepas. the Goose- 

 barnacle (Fig. 131), as mollusks; 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 accus- 

 toming its mind to the fact that these 

 animals are undoubtedly relations of the 

 well-known species of Crustacea. This 

 may be more intelligible 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, act've and free-swim- 

 ming, with a pear-shaped body, and three pairs of swimming-legs. This 

 larval stage it common to all the lower kinds of Crustacea and is termed 

 the Nauplius stage. But after several moults this larva fixes itself by 

 its head to some convenient object , and now the skin begins to secrete 

 the calcareous covering, which consists of several plates completely hiding 

 the animal, and only allowing the delicate legs to protrude from a slit- 



Fig. 168. Balamus perforatus, on 

 a roek, 1 / 2 nat. size. 



