189 
ly cylindrical ; -@ circular’ or’ ginichlanaad called the disc, 
bordered by one or more series of tentacles, and an opening or 
mouth at the centre of the disk: internally, @ visceral cavity 
which is closed below ; no distinct vascular system ; an imperfect 
nervous system or none ; no senses but that of touch; no distinc- 
tion of ser. The mouth is a simple opening through the fleshy 
- disc, without organs of manducation, and it is the only exit from 
the visceral or internal cavity.. The body, and in most instances, 
the. tentacles also, are expanded by means of water received 
_ from without, mostly through the mouth, and which is ejected 
on contraction. Instead of having gills,* the whole surface of 
_ the animal, inside and out, takes part in the function of aération, 
_ through the air of the exterior and interior waters. 
-The polyp in its simplest form (see figures 2 and 3, page 197) 
is essentially then merely a fleshy bag, or tube of flesh, closed 
at one end ; and within the walls of this tube nearly all the pro- 
cesses of life are carried on. ‘There is no division of labor—no 
separate organs for different functions, as in the higher animals ; 
» nutrition, aération, and reproduction, belong equally, as far as 
ean be discerned, to all parts of the simple ‘structure. With an- 
imals of this kind it would seem to be of little importance which 
side of the bag was innermost; and it is a fact with the Hydras 
that they may be turned insite out, and still eat, digest, and pe 
form all the functions of life as esfores 3h 
iikibs Only the lower grade of polyps have the extreme simplicity 
Bors described... As a first step in advancement, the upper part 
of the visceral cavity becomes partially separated from that below 
and serves as a'stomach. 'The second step is more important ; it 
consists in a localization of the function of reproduction, in ad- 
dition to the farther perfection of the stomach, The stomach, a 
simple tube, occupies the centre of. the visceral cavity, being at- 
tached to the disc immediately below the mouth. The space 
in the visceral cavity outside of the stomach and below it, is di- 
- Vided into vertical compartments by a radiate series of fleshy 
lamelle or plates, part of which bear eee cords on the mar- 
gin,” and part, clusters of ‘ova. 
* Organs somewhat resembling gills are found in the family Zoanthide ; but 
none have been detected in other polyps 
igs te comemcam Vol. II, No. a 1846. 25 
