Crabs, Lobsters, Shrimps, Barnacles etc. 



75 



eyes , and the tail of the fish by means of their mouth-appendages and 

 the sharp sickle-shaped legs (7 pairs); or they are attached within the 

 gills or the throat of the fish and suck its blood; they may attain the 

 length of a couple of inches. They fix themselves so firmly that no 

 endeavours of the tortured animal can dislodge them. The females carry 

 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, 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), are 

 marine forms. Phronima, the Hermit-screw (Fig. 168), is especially 

 interesting. It is a perfectly transparent 

 pelagic form, 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 

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

 form rapid strokes, which propel the animal 

 together with its house through the water. 

 This invertebrate Diogenes uses its trans- 

 parent 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 jelly-fish and other "pelagic forms" especially 

 in the months of winter and spring, and will be found occasionally in 

 tank NT. 20. 



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

 of the Cirripedia ("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, the Acorn-bar- 

 nacle (Fig. 169) 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 forms of Crustacea. This 

 may be more intelligible w r hen 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- 



Fig. 168. Phronima seden- 

 taria in its barrel. 



Fig. 169. Balanm perfpratus on 

 a rock, */2 na t- size. 



