10 



formed, and at which the branch is finally thrown off. From 

 the regularity of the process by which this is effected, the 

 well denned scar left behind, and the central pith not 

 being exposed, there is but little doubt left on my mind 

 that each branch is thrown off by a vital process, similar to 

 sloughing in animals, or the shedding of leaves in trees, 

 and is not washed off accidentally by the sea. That very 

 stout and rigid species the Sertularia nigra is exposed to the 

 same violence, rendered more effective by its rigidity, and 

 yet is not liable to have its pinna? destroyed in a similar 

 manner; and it is not unfrequently the case that in this last 

 species the pinna? are torn or twisted off at various lengths, 

 but such cases are not at all like the loss in the Th. thuja 

 or T. articulata. The terminations of the stem and pinna? 

 are said to be open, during growth, which, however, does not 

 ■appear to be the case ; but if care be not taken to keep 

 the specimens healthfully alive, or if they should be placed 

 in fresh water, these parts from being young and delicateJy 

 formed, readily rupture and the granular pulp is forced 

 through the apertures. To such an extent is this sometimes 

 the case, that through the whole polypidom scarcely a 

 trace of the pulp remains. This seems to arise from the 

 irritability of the horny sheath, for that it is irritable there 

 is sufficient proof. 



There is an opinion entertained by some physiologists, 

 that the external horny sheath is extravascular and con- 

 sequently not endowed with vitality. It is considered by 

 them as an exudation from the granular pulp. To this, 

 however, my observations do not enable me to agree. Their 

 mode of growth, the formation of their cells, and the 

 manner in which they cast their branches tend against 

 such a conclusion. In the formation of the cells, their 

 apertures are always closed at first, and the polypes fully 

 developed before they are opened ; the polypes, in this 

 enclosed state, do not press against the upper part of the 

 horny covering, but lie towards the centre of the cell, yet 

 the upper portion gets thinner and finally opens about the 

 middle and falls in, in the form of a funnel. Jf this part 

 was ruptured by the pressure from within, the torn margins 

 would have been forced outwards, rather than hare been 

 inverted as is always the case. This mode of opening a 

 cell is nearly alike in all the genera, yet they all afterwards 

 assume their specific differences — differences so remarkable 

 and constant as to leave no doubt but that they are the 

 result rather of specific actions than of an accident. The 

 peduncles of the cells of the Campanularia are naturally but 

 ringed at intervals, yet the animal possess a power to 

 corrugate them throughout their whole extent, and does 

 it at one time and not at another. When the extremities 



