RADIATA. 27 



they are drawn together by the contraction of the arms. Most of theses 

 ])rojections support a sliort hair, some are armed with a thorn, and othei^s 

 support a very long hair ending with a spur composed of several thorns 

 around a pear-shaped mass. See the last edition of the Regne Animal, 

 Zoophytes, fl. 64. 



It is evident that the ITydrse, like the Medusae or sea nettles, have a 

 stinging power, judging from the manner in which their prey is paralyzed 

 when seized. Worms which will live and move for some time when cut 

 into fragments, die instantly when seized by a hydra : and if a worm which 

 has been seized is taken from the animal before it is swallowed, it does not 

 revive. When a minute animal comes within reach of one of the arms, it 

 is seized and swallowed, and this sometimes happens to young fishes. 

 They sometimes even swallow each other, but the swallowed individual is 

 cast out again unhurt. The uniformity of structure is proved by the fact 

 that if the animal is turned inside otit, the food can be digested by what was 

 once the outside. 



The genus Hydra has been named after the fabulous monster of antiquity 

 bearing the same name, because parts cut away will be reproduced; and 

 under favorable circumstances, when an individual is divided into several 

 parts, each part will become a perfect animal. It is probable that Hydra 

 grisea {pi. 75, ßg. 21), //. fusca {fig. 22), and H. viridis {fig. 23), are 

 varieties of but a single species. 



The order Hydroida, according to Mr. Dana, contains the following 

 families : 



1. HydridoB. Not coralligenous. 



2. Sertularidw. Coralla corneous. Sertularia abietina {pi. 75, fig. 33). 

 S. polysonalis {fig. 30). S. operciilata {fig . 34). Planularia falcata {fig. 

 31). Thuia/ria thuia {fi^. 32), all Em'opean. . 



" In a single specimen of Plumularia angulosa collected by the author in 

 the East Indies, there are about 12,000 polyps to each plumose branch ; and, 

 as the whole zoophyte, three feet long, bears these plumes on an average 

 every half inch, on opposite sides, the whole number of polyps is not short 

 of eight millions ; all the oflspring of a single germ, and produced by 

 successive budding." — Dana. 



3. Campanularidöß. Coralla corneous, calicles pedicillate. 



4. Tabularidce. Coralla tubular and corneous. Tahularia indivisa {pi. 

 75, fig. 42) (Atlantic and Mediterranean), attached to stones in deep water, 

 T.coronata {pi. I^^fig. 29) (Northern seas). Stem one third of a line in 

 diameter. In this genus the tentacles are not retractile. 



The remaining orders of the class Colenterata were called Acalephse by 

 Cuvier, from the Greek word axaXrjpvj a nettle., from the stinging quality 

 which many of these animals possess. Blainville gave them the expressive 

 name of Arachnoderma., from a^a^viov a cobweb., and (Japnia the skin., from the 

 extreme tenuity of the tissues. They are also termed Medusa^, sea-nettles, 

 stang-fishes, sea-jellies, and jelly-fish. They are found floating in all seas, 

 particularly those of the tropics ; their size varies from one sixth of a line to 



231 



