SWIMMING ANIMALS. 45 



siderably exceed a foot. Medusae belong to that great 

 group of animals known as Zoophytes, which includes 

 the Polypes, Sea-Aneniones, and Corals. They afford 

 an example of the so-called "alternation of genera- 

 tions;" being themselves developed by the division of 

 a fixed polype into a number of saucer-like sections, 

 which become free and swim away, and in turn lay 

 eggs, again developing into fixed Polypes, like the 

 original parent. 



Our next illustration is taken from the Crustaceans, 

 among which the Lobsters and some of the Crabs are 

 expert swimmers. In the Lobster and Cray-Fishes, 

 where the tail is long, and furnished with five hinged 

 and paddle-like plates, the most rapid motions in the 

 water are effected by suddenly bending the tail beneath 

 the body, and thus driving the creature forcibly back- 

 wards by the recoil of the water. Prawns and Shrimps 

 have a similar mode of swimming ; but those Crabs 

 which, like the " Fiddlers," are free swimmers, have the 

 terminal joints of the fifth pair of legs (and sometimes 

 also those of the three next pairs) developed into flat 

 paddles fringed with hairs. These claws are thus 

 quite different from the pointed claws of the common 

 Shore-Crab. Since Crabs and Lobsters breathe by 

 means of gills, they may be safely regarded as primitive 

 swimmers ; those species which, like the Land-Crab, 

 are terrestrial having acquired this habit, and thus 

 having to put up with the inconvenience of keeping 

 their gills constantly moist. We cannot take leave of 

 the Crustaceans without mentioning the Barnacles, as 



