COMPENDIUM. 275 



from decay, it may be, into a tenacious ropy matter, where most abundant, 

 as in the Tubularia, when it is profusely discharged ; the transparence of 

 the large tubular parts of some of the Sertulariae exposes the pith, like a 

 continuous slender thread, originating below, traversing all the skeleton, 

 and terminating with the development of the hydra at the extremity. 

 Though having had repeated occasion to refer to it as an important part 

 of the organization of zoophytes, we must admit that all I have said is 

 hypothetical, for its precise use is unknown. But we may justly infer, 

 from evident facts, that it undoubtedly contributes to the vigour, life, and 

 increment of zoophytes, as they perish inevitably on its permanent decay. 

 Likewise, there is probably some immediate connection between the ele- 

 ments of the hydra and the pith, all as previously intimated. In the Tu- 

 bularia, these elements certainly reside below, from which, by the regene- 

 rative energies of the species, new hydra? will be successively developed, 

 as the stem is reduced shorter and shorter, by repeated sections, the hydrse 

 being always produced from what remains next the root. 



As a general principle, it may be advanced that the increment of the 

 inorganic parts depends on the subsistence of the organic. The connec- 

 tion of the hydra with the pith, is immediate and obvious in the Tubularia 

 and Sertularia. But the existence of the pith itself, and the reciprocal 

 connection of the hydra with any similar intermediate substance, is not 

 ascertained in the Flustra and other ascidian zoophytes. There the natu- 

 ral arrangement seems to be somewhat different. The correspondence of 

 the parts is not yet discovered. Neither is it clear what precise connec- 

 tion subsists among the pectinate hydra? of the carnose zoophytes, or the 

 nature of the polyparium itself, so that the reciprocal influence of the parts 

 cannot be described. A medullary stump generally or always accom- 

 panies the deciduous head of the Tubularia on separation. 



The great variety of external configuration exhibited by the inor- 

 ganic portion, has afforded facilities for the arrangement of zoophytes by 

 systematic naturalists. 



But it is singular that so little aid seems to be derived as appreciable 

 by our senses, from the organic parts. In as far as we can determine, the same 

 species of hydra may be combined with an hundred different inorganic struc- 



