90 



hollow enlargements at the angles, which form their specific 

 differences. So extensive is this effusion occasionally, that 

 I hare known ten inches of cells in formation at o"ne time. 

 This form of growth fully explains the great length and the 

 comparatively narrow extent to which this species attains. 



In some, and indeed in almost all cases, where the length 

 of the encrustation is great, the longitudinal lines of the 

 cells have a gently arched direction outwards. This arises 

 from the manner in which the lateral increase is effected, 

 and which may he termed the intercellular increase. As 

 the cells lie in their linear direction they gradually get 

 wider as they get more dislant from the centre of growth. 

 In this manner every eighth or tenth cell has so far increased 

 in size, that two smaller ones are formed on its extremity ; 

 these also increase in breadth, and hence a continued inter- 

 cellular enlargement takes place. The outer rows of cells 

 are. thus necessarily forced into the arched form, from these 

 internal wedges. This is the chief if not the only mode of 

 lateral increase. If any obstruction is offered to the longi- 

 tudinal growth, such as an orifice in the frond or any ine- 

 quality of the surface, the cells will take a circuit and meet 

 on the other side. If however the polypidom be injured 

 laterally, a small quantity of the pulp will be effused there ; 

 and the cells formed in it will be in the direction of the 

 effusion, or at right angles to the original source. So that 

 the direction of the lines of (he cells, is indicative of the 

 point from which the pulp was effused. In the Hairy Sea 

 Mat ( Membranipora pilosaj a lateral increase takes place 

 differently, but showing the same fact, that a calcareous 

 deposit takes place in the pulp, and is but a continuation of 

 the same process that caused the effusion of the matrix. 



fn the calcareous species, such as Cellepora pumicosa, 

 Eschar a, &fc, the formation of the cells is equally apparent; 

 but changes occur in the character of the cells after they have 

 acquired their specific markings, which tend still further to 

 prove that the polypidoms are organic and liable to similar 

 changes incident to other organic structures. In the encrust- 

 ing calcareous species, the direction of growth appears to 

 be diffused on all sides of the cells, and hence they generally 

 grow in circumscribed patches. Alter the pulp has been 

 effused, the lime is deposited in a similar manner to that des- 

 cribed above, differing, of course, in some measure in the 

 different species; in the Hairy Sea Mat for instance, the 

 cells pass through a series ol changes, each of which 

 is similar to the perfect cells of other species. After the 

 pulp has been effused and the cells perlectly formed, the 

 calcareous deposit is not trom that time suspended, for after 

 that poriod they considerably alter in appearance. The cells, 



