Crabs. Lobsters, Shrimps, Barnacles, etc. (Crustacea). 



such forked limbs and for this reason are probably descended 

 from animals resembling these Opossum-shrimps. In the tail-fan 

 these animals carry a pair of curious organs which were for a long 

 time considered as ears, but are now recognized to be balancing 

 organs (Statocysts). 



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

 reader perhaps knows the common Freshwater Shrimp {Gammarus 

 pulex), hve in the sea. Phronima, the Hermit-screw (fig. 163) is 

 especially interesting. It is a perfectly 

 transparent pelagic animal, and curiously 

 enough makes use of the young Pyrosonia 

 (see p. 100) 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. ^'g- ^^3- . Promma scdcn- 



■T^u- ■ i u J- TA- -J. X fa)-ia in its barrel. 



Ihis invertebrate Diogenes uses its trans- 

 parent 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 months of winter and spring, 

 and will be found occasionally in tank Nr. 20. 



Less pleasant is the acquaintance which the visitor to the 

 Aquarium makes with the marine Isopods and those who know 

 their relations the wood-lice will expect nothing very pleasing 

 from them. A large number of the marine Isopods have taken 

 to a parasitic mode of life and many of these forms (the so-called 

 Fish-lice) have for their victims (or, as one eupemistically ex- 

 presses it, hosts) the fishes which live in the aquarium tanks. 

 Parasitism is always a vice, but here we meet with it in a par- 

 ticularly repulsive form, so that the visitor will not be sorry if no 

 fish are visible at the moment with these blood-suckers attached 

 to head, eyes, mouth, throat, gills or tail. The parasites are of 

 about two inches in length and belong to the genera Anilocra 

 and Cymothoa. They fix themselves with fourteen sharp sickle- 

 shaped claws so firmly that the tortured fish strive in vain to rub 

 them off. Levelling justice has, however, happily seen to it that 

 on these parasites others prey, in the manner of the wellknown 

 rhyme: — ^-^ ^^^^^ ^^^.^ j-^^j^ fj^^^ 



Upon their backs to bite 'em, 

 Little fleas have lesser fleas, 

 And so at infinitum. 



The rich intake of nourishing fish-blood gives the Isopods 

 material for the production of a numerous brood, which the mother 



