RADIATA. 27 
they are drawn together by the contraction of the arms. Most of these 
projections support a short hair, some are armed with a thorn, 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 Régne Animal, 
Zoophy tes, pl. 64. . 
It is evident that the Hydre, like the Medusz 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 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 several 
parts, each part will become a perfect animal. It is probable that Zydra 
grisea (pl. 75, fig. 21), H. fusca (fig. 22), and H. viridis (jig. 23), are 
varieties of but a single species. 
The order Hydroida, according to Mr. Dana, contains the following 
families : ; 
1. Hydride. Not coralligenous. 
2. Sertularide. Coralla corneous. Sertularia abietina (pl. 75, fig. 33). 
S. polyzonalis (fig. 30). 8. operculata (fig. 34). Planularia faleata (fig. 
31). Thuaria thuia (fig. 32), all European. 
‘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 offspring of a single germ, and produced by 
successive budding.”— Dana. 
3. Campanularide. Coralla corneous, calicles pedicillate. 
4. Tabularide. Coralla tubular and corneous. TZabularia indivisa (pl. 
75, jig. 42) (Atlantic and Mediterranean), attached to stones in deep water, 
T. coronata (pl. 75, 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 Acaleph by 
Cuvier, from the Greek word axadAnpy a nettle, from the stinging quality 
which many of these animals possess. Blainville gave them the expressive 
name of Arachnoderma, from agexvov a cobweb, and dzpua the skin, from the 
extreme tenuity of the tissues. They are also termed Meduse, 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 
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