ON THE SKELETONS OF ZOOPHYTES. 
17 7 
substance is seen occupying the interior of the horny sheath, and, traversing like 
the pith of a tree the ramifications of the Polypary, the branches terminate in 
campanulate expansions for lodging the Polyps. When viewed in appropriate 
vessels under water, the habits of these interesting creatures may be satisfactorily 
studied; the mouth of the Polyp, when in quest of prey, is seen to be provided 
with numerous tentacula, endowed with the most exquisite sensibility, and rapidly 
moved in all directions; the gelatinous flesh is composed of minute granules, 
through which currents of globules are seen meandering slowly up the sides, and 
returning in an opposite course down the centre. The axillse of the branches at 
certain seasons are occupied by ovate deciduous vesicles, which play an important 
part in the generative process. In these operculated chambers the gemmules bud 
forth from the parent stem, and undergo the process of development; having- 
arrived at maturity, the opercula drop off, and allow the embryos to escape into 
the sea. The fleshy pith which occupies the centre of the stem and branches is 
enclosed, according to Lister,* in two distinct laminae; the internal layer invelops 
the general gelatinous flesh, and forms the membranous structure of the nutritive 
Polyps; the external layer lines the inner surface of the ramified Polypary, and 
expands with the growth of the organism to form the Polypiferous and ovigerous 
vesicles. The skeleton is therefore a secretion of indurated albumen, furnished by 
the tegumentary membrane; and as it shoots upwards, it assumes the form and 
direction prescribed to the individual species. When a branch in process of 
growth is examined with the microscope, it is seen to be open at its distal 
extremity, through which the secreting membrane of the horny sheath protrudes. 
When it has attained its specific length, the granular flesh of the newly-formed 
branch becomes gradually more and more dilated at its extremity, and the 
tegumentary membrane is observed to expand, and deposit corneous matter, to 
form a new cell; whilst the vesicle is in process of growth, a concentration of the 
vital action is going forward in the gelatinous flesh, a septum is first thrown across 
the bottom of the cell; next an outline of the future Polyp is seen through the 
diaphanous sheath, which subsequently acquires tentacula, and being thus per¬ 
fected in all its parts, the newly-formed Polyp rises from its cell, and expands 
its little arms in quest of nourishment to contribute its part to the nutrition of 
the Polypary of which it constitutes an integral part. 
The delicate lace-work of minute cells observed on the surface of Fusi, or seen 
spreading in retiform expansions upon the shells of Mollusca , are the cellular 
skeletons of zoophytes belonging to the order Uetiformia , of which the Sea Matt 
(Flustra carbesia ) affords a familiar example. When obtained fresh from the 
water, and attentively examined with a moderate magnifying power, the surface 
Philosophical Transactions. 
