SEA-NETTLES. 103 
from without with the cavity of the stem are by the mouths of 
the digestive tubes, which answer to the bodies of polyps. The 
food digested by these stomachal polyps is conveyed from their 
extremity into the cavity of the stem, from whence it is carried 
through the vessels of all the appendages, partly by the contractility 
of the walls of the stem, partly by the action of the cilia which 
line the vessels of the appendages. 
The polyps, or suctorial tubes, or stomachs, have no tentacles 
round the mouth. They consist of three portions; the external, 
very variable in form, the proboscis and mouth: the middle swollen 
portion, the digestive stomach, with dark streaks containing bile- 
cells: the terminal rounded portion, with thick cellular walls. At 
the base of the stomach, or sometimes immediately on the common 
stem, is the prehensile apparatus for the capture of prey. This 
usually consists, for each polyp, of a single long and thin thread 
with lateral subdivisions, which do not branch ; more rarely of sim- 
ple threads or shorter cylinders. This apparatus is always supplied 
with multitudes of thread-cells, which in the case of lateral acces- 
sories are grouped in very regular and constant forms, and are con- 
spicuous from their bright yellow colour. The sexual appendages 
have large swimming bells of the general medusan form. They 
consist of a bell-shaped mantle and vessels—and a nucleus, more or 
less conspicuous, which contains in its substance the sexual ele- 
ments, and is dependent from the vertex like the clapper of a bell. 
In some cases the medusan form of the mantle is in great measure 
suppressed, whilst in others it is quite complete, and here the sexual 
appendage is detached at an early period, as in certain hydroid 
polyps, and the sexual elements are developed afterwards: where 
the medusan form is not thus perfect, the contents of the sexual 
capsules, when detached, are found to be mature. The Diphyide 
are, according to Leuckart, all uni-sexual, but the observations 
of GEGENBAUER (Zettschr. f. wissensch. Zool. Vv. p. 313) shew that 
some at least have the organs of different sexes on different groups 
of the same stem: the Physophoride are all bi-sexual, in some 
(Stephanomia) the organs of the two sexes being on different pedi- 
cles, in others (Physalia) on the same pedicle. 
The organs of less general occurrence are the Bracts, Lamine 
or Covers, and the Feelers. The Bracts or Covers, more solid 
than the other organs, are for their protection: they contribute little 
