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 Diphyidce 

 are, according to LEUCKART, all uni-sexual, but the observations 

 of GEGENBAUER (Zeitschr. f. wissensch. Zool. v. p. 313) shew that 

 some at least have the organs of different sexes on different groups 

 of the same stem : the Physophoridce 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, Laminae 

 or Covers, and the Feelers. The Bracts or Covers, more solid 

 than the other organs, are for their protection : they contribute little 



