rOLYPIFERA. 



permeates the stem, which thus seems to be 

 nourished by the hundreds of little polypes 

 that are constantly fishing for food. 



At certain periods of the year, besides the 

 polype-bearing cells, other horny receptacles 

 are developed, called the ovigerous vesicles 

 (fig. 55, h), in which the ova are produced. 



The ovigerous vesicles are differently dis- 

 posed according to the species, sometimes 

 arising from the branches of the coralline, at 

 others from the axillte formed by their sub- 

 divisions ; their shape likewise is very various, 

 and sometimes they are covered with a little 

 operculum, or lid, which closes the orifice of 

 the vase-like vesicle during the maturation of 

 the reproductive gemmules, and at last opens 

 so as to permit their escape. These gemmu- 

 liferous urns are, however, deciduous, and 

 fall off after the development of the germs of 

 reproduction is completed. 



Such being the general structure of the 

 Sertularidae, we must now proceed to examine 

 more minutely their intimate organisation. 

 The stem of the Sertularian is composed of 

 two layers, of which the exterior (Jig. 55, b), 



Fig. 55. 



Diagram of Sertularian. 



a, inner or nutritive layer ; b, outer or tegu- 

 mentary layer ; c, oral tentacles of the polype ; d, e, 

 gemmules ; /, polypiforin external capsule ; h, ovi- 

 gerous cell. 



or tegumentary layer, is of a dense horny tex- 

 ture, while the internal, or nutritive layer (fig. 

 55, a), is of a soft pulpy character according 

 to the pattern peculiar to the species ; the tegu- 

 mentary layer expands at appointed distances 

 into the polype-cells (fig. 55, g) ; and it is from 

 this layer likewise during the reproductive 

 season that the ovigerous vesicles are deve- 

 loped. The nutritive layer (fig. 55, a) it will 

 be seen, not only lines the stem, but likewise 

 penetrates into the polype-cells, where it be- 

 comes continuous with the body of the con- 



VOL. IV. 



tained polype, the structure of which closely 

 resembles that of the Hydra ; it seems, in fact, 

 to consist of nothing but a stomachal sacculus, 

 the mouth of which is surrounded with con- 

 tractile tentacles, which are never, as errone- 

 ously stated by some writers, provided with 

 vibratile cilia, such as are possessed by some 

 more highly organised polypes. The nutriment 

 elaborated in the digestive sacculi passes into 

 the central cavity of the stem, in which an 

 evident circulation of globules is apparent, 

 somewhat analogous in its appearance to what 

 is perceivable in the Chara and other transpa- 

 rent vegetables. 



It is from the nutritive layer which lines 

 the ovigerous vesicles likewise that the repro- 

 ductive gemmules are developed. These 

 (fig. 55, d), as they gradually become separated 

 from the nidus in which they are formed, 

 retain their connection with the vital tissue 

 of the nutritive layer, by the intervention of a 

 kind of umbilical cord, until they are suffi- 

 ciently matured to allow of their escape. 

 When this period arrives each gemmule is 

 found to be covered over with vibratile cilia, 

 by the action of which it detaches itself from 

 the umbilical filament, and, escaping from the 

 reproductive cell, swims away into the sur- 

 rounding element. 



Here, by means of its cilia, it swims about, 

 having much the appearance of a polygastric 

 animalcule, until it finds a fit locality for its 

 development, when it settles down, and, 

 losing its locomotive organs, spreads out like 

 a film of jelly upon the supporting body. The 

 formation of its horny envelope then begins, 

 fibres of which are first extended like the 

 spreading root of a tree, so as to give a firm 

 hold upon the basis for support ; and then 

 the stem itself begins to shoot upwards, deve- 

 loping, as it ascends, the nutritive polypes and 

 the horny cells in which they are individually 

 lodged. 



In order to understand how growth is ac- 

 complished in these tube-clad zoophytes, it will 

 be necessary to refer once more to the preced- 

 ing diagram (fig. 55). The tegumentary layer 

 of the zoophyte (fig. 55, b) is at first quite 

 soft and expansible, the hard corneous matter 

 by which it is consolidated being afterwards 

 superadded to its texture. Whilst growth is 

 in progress, therefore, this outer layer shoots 

 upwards in conformity with the pattern to 

 which it belongs ; but whilst the top of the 

 tube retains its softness and power of growth 

 it is continually fortified below by the depo- 

 sition of the horny matter which gives it 

 solidity : growth can therefore only proceed 

 at the extremity of every branch where this 

 layer remains capable of further development ; 

 for no sooner is it solidified than it remains 

 permanently unchangeable. Hence it is that 

 these zoophytes differ so remarkably from 

 plants in the character of their arborescence : 

 in the latter the stem is increased by constant 

 additions to its thickness, but in the case of the 

 Sertularia no such thickening is possible ; so 

 that both stem and branches retain the same 

 diameter throughout, however much their 



E 



