114 BRYOZOA. 



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 pre- 

 sents the appearance of a stony mass in which the apertures of the 

 cells only are visible. 



It appears evident, therefore, that there is vitality in the sub- 

 stance composing these 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. 



(150.) The anatomy of these Bryozoa differs slightly from that 

 of Bowerbankia. The crown of ciliated tentacula is inserted into 

 the extremity of a kind of proboscis, which is itself enclosed in a 

 cylindrical retractile sheath. From the margin of the opening of 

 the cell arises a membrane equalling in length the contracted ten- 

 tacles, and serving to enclose them when the animal retires into its 

 abode. These appendages, thus retracted, are not bent upon them- 

 selves, but perfectly straight and united into a fasciculus, the 

 length of which is nevertheless much less than that of the same 

 organs when expanded. 



By the opposite extremity to that fixed to the margin of the 

 opening of the cell, the tentacular sheath unites with a tolerably 

 capacious tube, the walls of which are exceedingly soft and deli- 

 cate ; and near the point of their union we may perceive a 

 fasciculus of fibres running downwards to be inserted upon the 

 lateral walls of the cell : these fibres appear to be striated trans- 

 versely, and are evidently muscular ; their use cannot be doubted : 

 when the animal wishes to expand itself, the membranous sheath 

 above alluded to becomes rolled outwards, everting itself like the 

 finger of a glove as the tentacles advance. The muscular fasciculi 

 are thus placed between the everted sheath and the alimentary 

 canal, and by their contraction they must necessarily retract the 

 whole within the cell. 



The first portion of the alimentary tube is inflated, and much 

 wider than the rest ; it forms a kind of chamber, in which the 

 water set in motion by the vibration of the cilia upon the tentacles 

 appears to circulate freely. The walls of this chamber are ex- 

 tremely delicate ; the soft membrane forming them is puckered, and 

 appears traversed by many longitudinal canals united by minute 

 transverse vessels ; this appearance, however, may be deceptive. 



