BRYOZOA. H3 



calcareous crust moulded upon the surface of the body, being a 

 portion of the tegumentary membrane, which, by the molecular 

 deposit of earthy matter in its tissue, ossifies like the cartilage of 

 higher animals without ceasing to be the seat of nutritive move- 

 ment. It is evident, likewise, that what is called the body of the 

 Bryozoon constitutes, in fact, but a small portion of it, principally 

 consisting of the digestive apparatus. 



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 re- 

 mains sufficiently soft and flexible to obey the action of the mus- 

 cles inserted into it. 



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

 seems to be formed of a tomentous membrane, covered, especially 

 upon its outer side, with a multitude of cylindrical filaments disposed 

 perpendicularly 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. 



But the above are not the only arguments adduced by Milne 

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

 skeletons are held in vital connection with the animal. On ex- 

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

 material changes of form. 



This examination is easily made, since in many species the 

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

 rate 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 position 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 extremi- 

 ties of the latter. When examined in this manner, it is seen that not 

 only does the general configuration of the cells change with age, but 

 also that these changes are principally produced upon the external 

 surface. For instance, in the young cells of Eschara 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 



