132 PHYSIOLOGY. 



circular canal the fluid may also be seen flowing at one time in one direction, and then again in 

 the opi^osite. 



That the motion of the somatic fluid in all these cases is mainly caused by the impulse of 

 vibratile cilia, there can be but little doubt. Dujardin appears to have witnessed these cilia in 

 the stem of his Syncoryne decipiens ; ' and in the stem of Tuhularia indivim, where the cilia 

 clothing the lacunar channels and their relation to the currents were first pointed out by Dr. T. 

 S. Wright, ° they are very distinct, and can be demonstrated with the greatest ease in a transverse 

 section of the stem (PI. XXIII, fig. 7). By carefully applied pressure, I have succeeded in ejecting 

 the spadix in an inverted state through the proximal end of a detached sporosac of Tubidciria 

 iiidivi-m, when its internal surface being thus exposed, a rich clothing of very distinct actively 

 vibrating cilia has been brought into view (PL XXIII, fig. 10). 



It must, however, be admitted that in the greater number of hydroids the existence of endo- 

 dermal cilia has not been proved by direct observation. In most hydroids the tissues are too 

 opaque to atford a view of the delicate cilia which may clothe the cavities, while sections of the 

 stem will aflbrd none of that aid which we obtain from them in Ttibtdaria ; for the section is 

 accompanied in most other species by a collapsing of the cavity which would necessarily interfere 

 with the action of the cilia and remove them from observation. In every case where a satisfactory 

 view of any portion of the free surface of the endodenn in the Hydroida has been obtained, as, 

 for instance, where the transparency of the tissues is such as to afford no obstacle to a view of the 

 deeper seated parts, this smface is invariably — except in Hydra, where cilia appear to be really 

 absent — seen to be clothed with vibratile cilia. Thus, in certain meduste the cilia may be easily 

 seen vibrating along the walls of the radiating and circular canals. 



It is probably, then, a nearly universal fact that the free surface of the endoderm in the 

 Hydroida is ciliated, and that the fluid in contact with this surface is kept in motion by the 

 impulse it receives from the cilia. 



It uHist not, however, be overlooked that the contractility of the walls has its share in this 

 motion. The fluid contained within the gastric cavity of the hydranth may, indeed, be frequently 

 seen to be forcibly expelled from this cavity by its contraction into that of the coenosarc — the 

 mouth of the hydranth being at the same time kept closed — and then again drawn back into the 

 gastric cavity, when the contraction which had expelled it is succeeded by an expansion. 



As every portion of the somatic fluid is thus successively brought in contact with the walls 

 of the somatic cavity, it yields to the tissues by a process of direct absorption the nutriment 

 necessary for their growth and maintenance, and receives from them such portions of their sub- 

 stance as in the performance of their vital functions had become effete. 



That the formation and growth of the tissues is intimately connected with cell-formation 

 is rendered obvious by attending to the phenomena which accompany rapid growth among the 

 Hydroida. 



A healthy colony of Obclia dicliotoma was placed in a jar of sea-water. In a few days most 

 of the hydranths had disappeared, but adventitious branches had begun to be sent out in great 

 profusion from various parts of the surface. These branches elongated themselves with 



' ' Ann. des Sc. Xat.,' tome iv, 1845, p. 275. 

 - ' Proc. Roy. Ph. Soc. Edinb.,' 1855-50. 



