Page:The Zoologist, 1st series, vol 1 (1843).djvu/234

206 power of any vital action, and are liable to no alteration but what may be produced by a chemical or mechanical change in their ultimate particles. Thus, in the Hydroidae, the central and living pulp is enclosed in an exuded and dead sheath; and, in the Asteroids), an inorganic axis is enclosed in the centre of the living mass.

The solid or horny part of the Hydroidae is sometimes said to be formed by an exudation from the surface of the granular pulp, and at others, by the pulp distending the horny cuticle of the gemmulc, and extending it into all the ramifications and branches that this beautiful order assumes. That the sheath depends for its existence on the pulp, or that it is intimately connected with it in a physiological relation, is evident, for they are simultaneous in their growth; in proportion to the growth of the pulp is the extension of the sheath, always bearing a relative proportion to each other. Thus they may be said to be mutually dependant, and together to form the perfect animal. But whether the sheath be a dead and an inorganic exudation will be best seen by an examination of its mode of formation, with the production of the cells, ovarian vesicles, and other circumstances connected with them.

a, b, c, d, e, Different stages in the development of the polype. f, The infundibuliform opening in a young cell of the Knotted Sea-thread. g, Sections of new and old purls of the sheath of Sertularla Abietina.

The mode of growth of the branches, and formation of the cells and vesicles, is nearly alike in all, being but very slightly modified in the different species, so that an examination of one may suffice for the whole order. That common species, Sertularia pumila, may be