ORGANIZATION OF CELL. 469 



body of the Polyzoon constitutes, in fact, but a small portion of it, 

 principally consisting of the digestive apparatus. 



(1215.) As to the operculum destined to close the entrance of the 

 tegumentary cell, it is merely a lip-like fold of the skin, the marginal 

 portion of which acquires a horny consistence, while at the point where 

 it is continuous with the general envelope it remains sufficiently soft 

 and flexible to obey the action of the muscles inserted into it. 



(1216.) The tegumentary sac, deprived of its carbonate of lime, seems 

 to be formed of a tomentose membrane, covered, especially upon its 

 outer side, with a multitude of cylindrical filaments disposed perpen- 

 dicularly to its surface and very closely crowded together. It is in the 

 interstices left by these fibres that the calcareous matter appears to be 

 deposited ; for if a transverse section be examined with a microscope, 

 the external wall is seen not to be made up of superposed layers, but 

 of cylinders or irregular prisms arranged perpendicularly to the axis of 

 the body. 



(1217.) But the above are not the only arguments adduced by Milne- 

 Edwards in confirmation of this view of the mode in which these 

 skeletons are held in vital connexion with the animal. On examining 

 the cells at different ages, it is found that they undergo material changes 

 of form. 



(121 8.) This examination is easily made, since in many species the 

 young spring from the sides of those first formed, and do not separate 

 from their parents ; each skeleton therefore presents a long series of 

 generations linked to each other, and in each portion of the series the 

 relative ages of the individuals composing it are indicated by the posi- 

 tion which they occupy. It is sufficient therefore to compare the cells 

 situated at the base, those of the middle portion, those of the young 

 branches, and those placed at the very extremities of the latter. "When 

 examined in this manner, not only is it seen that the general con- 

 figuration of the cells changes with age, but also that these changes are 

 principally produced upon the external surface. For instance, in the 

 young cells of Eschar a cervicornis (the subject of these observations), the 

 walls of which are of a stony hardness, the external surface is much 

 inflated, so that the cells are very distinct, and the borders of their 

 apertures prominent; but by the progress of age their appearance 

 changes, their free surface rises, so as to extend beyond the level of the 

 borders of the cell, and defaces the deep impressions which marked 

 their respective limits. It results that the cells cease to be distinct, and 

 the skeleton presents the appearance of a stony mass, in which the aper- 

 tures of the cells only are visible. 



(1219.) It appears evident therefore that there is vitality in the sub- 

 stance composing the stony walls ; and the facts above narrated appear 

 only explicable by supposing a movement of nutrition like that which is 

 continually going on in bone. 



