ON POLYPS. 49 



species ; and as it is peculiar to zoophytes of this description, 

 we shall dwell upon it at some length. At certain periods of the 

 year, besides the ordinary cells which contain nutritive polyps, 

 others are developed from different parts of the stem, which may be 

 called female or fertile polyps, although usually simply termed 

 the vesicles. The cells of this kind are much larger than the 

 nutritive cells, and of very different forms ; they are moreover 

 deciduous, falling off after the fulfilment of the office for which 

 they are provided. They are produced in the same manner as 

 the rest of the stem, by an extension of the tegumentary mem- 

 brane, (fg. 14,6,) which, as it expands into the form of the cell, 

 becomes of a horny texture ; it may be traced, however, over the 

 opening of the cavity, where it sometimes forms a moveable oper- 

 culum. The cell being thus formed from the expansion and subse- 

 quent hardening of the tegumentary membrane, it remains to explain 

 the origin of the reproductive germs which soon become developed 

 in its interior.* These are seen to spring from the inner or nutritive 

 layer of the polyp (a), to which they are attached by pedicles, re- 

 garded by authors as fulfilling the office of umbilical cords during 

 their early growth. As the germs expand, they gradually advance 

 towards the opening of the cell, where, as they are protruded, 

 each becomes covered with a layer derived from the tegumentary 

 membrane ( f) which closed the orifice, and protruding externally, 

 has very much the form and appearance of a young polyp, for 

 which indeed it has often been mistaken. We are assured, how- 

 ever, that this supposition is erroneous, and that the polypiform 

 bodies are only external capsules inclosing the real germs (e), 

 from which young polyps are to be formed.^ On tearing open 

 one of these capsules when the included germs are ripe, the latter 

 are seen to be rounded grains of a gelatinous appearance, covered 

 externally with minute cilia, which, like those of the gemmules of 

 the sponge, enable them to swim about at pleasure in search of a 

 proper locality whereon to fix their permanent habitation. These 

 ciliated gemmules are highly irritable, and frequently contract 

 their bodies into different shapes during their progress through the 

 water ; but at length, when about to fix itself, each gemmule becomes 

 flat and circular, and assumes a radiated appearance, resembling a 

 minute grey star, having the interstices between the rays filled with 



* Lcefling. Miiller's Archives, 1826. Lister, Loc. cit. 



t Professor Grant, Edinb. New Philosoph. Journal, 1827. Observations on the 

 spontaneous motions of the Campanularia Dichotoma, &c. 



