ercise no influence on the growth or development of the mass, except so far as they are the points through which nutrition is taken in. To separate therefore these parts by so great a physiological separation as life and death, is to do violence to Nature. One plan of life governs both, as well in their specific formations as in their mere growth. Many species assume some varieties of general form, depending on the localities in which they grow. The Plumularia cristata, for instance, in favourable situations will trail along on sea- weed, and send forth elegant plumes from one to four inches in length; but near the shore, in less favoured spots, they scarcely reach a quarter of an inch in length, and are exceedingly pale and slight; sometimes they rise from stones in a single plume, and at others have an irregular and branched appearance; but, in all, the cells and ovarian vesicles retain their specific differences: but as the polypes and the granular pulp are not in actual contact with these parts at the time in which the specific differences are formed, in what way are these differences so unerringly formed, unless by a power in the sheath itself? The whole of the observations I have been able to make on these creatures can be best explained by supposing a vital relationship existing between the pulp and sheath, and the irritability of the latter can hardly be explained in any other way. It is not more difficult to conceive the vitality of the sheath of the Sertulariadæ than of the teeth of animals. Teeth were almost always considered to be endowed with vitality, yet, till the late researches of Nasmyth and Owen, we had no decided proofs of it: their structure alone is now a sufficient proof, but this is still further strengthened by their capability of being injected. But though the polypidoms of the Hydroidæ have not been injected, nor in them have been detected traces of vascularity or a cellular structure, yet the circumstances under which they exhibit irritability are proofs sufficient to indicate a vital character. That the sheath is not a mere distension of the cuticle of the gemmule every one will allow, since its thickness is nearly alike in all parts, and the growth of the whole is sometimes so rapid that simple expansion would produce a rupture.
The opinion of Dr. Roget that "polypi are for the most part attached to some inorganic shell or base, which may be either of a horny or calcareous nature," seems therefore to be erroneous: 1st. From the manner in which the growth of the whole, and formation of the cells take place. 2ndly. From the internal alteration of the structure of the sheath, through age. 3rdly. From the shedding of the ovarian vesicles and branches. 4thly. From the irritability of the
