SEETULAEID^S. 93 



(218.) The general stem of the polypary is entirely filled with a 

 fleshy substance exactly resembling in its nature the tissue composing 

 the body of the polyp, whereby all the individuals belonging to the 

 common stock are brought into communication with each other. In- 

 ternally it seems to be hollow, and to contain a fluid, in which nume- 

 rous globules may be observed in active motion. It is from this central 

 fleshy substance that the buds or lateral offsets derive their origin. 



(219.) On examining attentively the stem of a living Sertularia, the 

 globules are all seen to follow each other in distinct currents, and 

 sometimes may be observed to move in opposite directions in the same 

 branch : on arriving at the bifurcation of a stem, some seem to stop, 

 whilst others continue their course to the right or left. If the branch 

 of a living polypary, while in a state of activity, be divided and slightly 

 compressed, the globules that escape from the cut extremity still con- 

 tinue their movements for some considerable time, somewhat after the 

 manner of zoosperms ; and as this kind of motion, when observed ex- 

 ternally to the polypary, resembles very closely that which they exhibit 

 in its interior, it is apparently not dependent upon any pressure from 

 the walls of the general fleshy substance, but seems to be inherent in the 

 globules themselves. The general movement of the fluid contained in 

 a branch, however, more especially as relates to its direction, depends 

 upon the pressure exercised by the polyps ; so that if several individuals 

 on one side of a branch contract simultaneously, they sometimes even 

 force the contained liquid through the mouths of those upon the oppo- 

 site side. 



(220.) It has been generally stated that the living pith exudes from 

 its surface the horny matter which, by its concretion, forms the tube 

 or external skeleton investing the whole ; the accuracy of such a sup- 

 position, nevertheless, may well be questioned. We have already seen, 

 in the Tubipora muswa, that the calcareous tube investing that polyp 

 was produced by the interstitial deposits of earthy matter in the mem- 

 brane that originally constituted its outer case. In the tribe of zoo- 

 phytes we are now speaking of, we shall find the exterior tube to be 

 formed in a way precisely similar. On referring to the diagram 

 (fig. 45), the mode of its growth will be rendered intelligible. The 

 soft part, or living axis of the polypary, is seen to be contained in two 

 distinct layers : the inner one, a, being continuous with the digestive 

 sac of the polyp, and immediately embracing the granular matter, seems 

 to be the special seat of the nutritive process ; the outer or tegumentary 

 layer, 6, after leaving the tentacula, may be traced down the sides of 

 each polyp to the bottom of the cell, where its course is arrested by a 

 slight partition, at which point it turns outwards, lining the interior of 

 the cell as far as its margin, where, as in the Tubipora, it is seen to 

 be continuous with the horny matter itself. It is this tegumentary 

 membrane, then, which forms by its development the entire skeleton. 



