Crabs, Lobsters, Shrimps, Barnacles, etc. 



75 



about their eggs in a special brood-pouch, on the under surface of their 

 body, till the young- ones are hatched. A curious fact worth noticing- 

 is that all females have in their youth been males ^ the new born 

 animals are not divided into the two sexes but, fig-uring- 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. 168), is especially 

 interesting. It is a perfectly transparent 

 pelagic animal, and curiously enough makes 

 use of the young Pyrosoma (see p. 85) 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 

 liind end of its body, the legs of which per- 

 form rapid strokes, which propel the animal 

 together with its house through the water. 



Fig. 168. Phronima sedeii' 

 taria iu its barrel. 



This invertebrate Diogenes uses its trans- 

 parent tub as nursery too, keeping the young- 

 there for some time after they arc hatched. 



It is caught on the surface of the sea, together with jelly-fish and other 

 « pelagic animals » especially in the months 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 Cirripeda{'^ Tendril-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, tlie Acorn-bar- 

 nacle (Fig. 169), and Lepas, the Goose- 

 barnacle (Fig. 131), as mollusks; and it 

 was not till much later tliat their early 

 stages, and their anatomy, revealed the 

 fact that they belonged to the Crustacea. 



The general public will tlierefore 

 also experience some difficulty in accus- 

 toming its mind to the fact that these 

 animals are undoubtedly relations of the 

 Avell-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 de- 

 generation. 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 hirval stage is 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 com- 



iter 



Fig. 169. Balanus perforatus on 

 a rock, '/2 nat. size. 



