206 



Zoophytes, 



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 en- 

 closed in an exuded and dead sheath ; and, in the Asteroidae, an in- 

 organic 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 gemmule, 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 re- 

 lation, is evident, for they are simultaneous in their growth ; in pro- 

 portion 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 exuda- 

 tion will be best seen by an examination of its mode of formation, 

 with the production of the cells, ovarian vesicles, and other circum- 

 stances connected with them. 



/ e d 



a, b, c, d, €, Difiereat stages in the development of the polype. /, The infundibuliforra opening in 



a young cell of the Knotted Sea-thread. g. Sections of new and old parts of the sheath of Sertularia 

 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 



