RADIATA. 27 



they are drawn togetlier "by tlie contraction of the arms. Most of tliese 

 projections support a short hair, some are armed witli a tliorn, and others 

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

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

 Zoophytes, jy^. 64. 



It is evident that the Hydras, like the MednstB or sea nettles, have a 

 stinging power, judgin^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 M'orm 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 out, 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 sevei-al 

 parts, each part will become a perfect animal. It is probable tliat Ilijdra 

 grisea {jjI. 75, fg. 21), //. fusca {fig. 22), and 11. viridis {fig. 23), are 

 varieties of but a single species. 



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

 families : 



1. IIydrid(£. ]Srot coralligenous. 



2. Sertularidce. Coralla corneous. Sertularia abieiina {jjl. T5, fig. 33). 

 S. 2)011/20716^18 {fig. SO). /S. operculata {fig. 34:). Plamdaria falcata {fig. 

 31). Thuiaria thuia {fig. 32), all European. 



" In a single specimen of Plumularla angidosa 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 offspring of a single germ, and produced by 

 successive budding." — Dana. 



3. CampanidaridcG. Corall| corneous, calicles pedicillate. 



4. Tabidaridce. Coralla tubular and corneous. Talularia indivisa {pi. 

 Y5, fig. 42) (Atlantic and Mediterranean), attached to stones in deep water, 

 T. coronata {j>I. 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 Acalephai oy 

 Cuvier, from the Greek word av.ak-f\yf, a nettle., from the stinging quality 

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

 name of ArachnoderTna., from a^ax^'o^ ^ cobweb.^ and 5sp,aa the sMn.^ 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 



