HYDROIDEA. 21 



represented in figure 6. The circle of tentacles is commonly sym- 

 metrical, yet is sometimes oblique. In the Tubularia group, the 

 tentacles are often short and sluggish, and are in one or more series, 

 or irregularly scattered. The disk is prolonged into a high cone, as in 

 figure la, and is tipped with a row of oral tentacles immediately about 

 the mouth opening. The tentacles of the Tubularise and Campanu- 

 laridse, are described as differing from those of the Sertularidse and 

 Hydra, in not being properly tubular organs. 



The stems and branches of these zoophytes are tubu- 



V fi 



lar ; and the stomachs of all the several polyps which 

 are simple cavities directly beneath the mouth com- 

 municate more or less freely with one another through 

 this common tubular axis, which ramifies from the 

 main stem into all the branchlets. Thus the polyps of 

 a cluster are united, not only by their external envelope, 

 but also through this internal communication. The 

 annexed figure exhibits this character in one of the 

 Campanularidae from the Feejee Islands ; and the same 

 is seen in the other compound Hydroidea. 



The axis is described by some as pulpy or medullary. In the 

 author's examinations of one of the Sertularidae (fig. 9), a vibrating 

 motion of the contents of the tubular axis was distinctly observed, 

 and the pulp, which had a greenish tint, appeared to have been de- 

 rived in part at least from the digested food of the stomach. The 

 investigations of J. J. Lister,* since seen, confirm this opinion. The 

 pulpy fluid was found by this able observer to vibrate occasionally 



* J. J. Lister, Philosophical Transactions, 1834, p. 369, with fine illustrations on 

 plates ix. and x. 



We quote the following from his very interesting observations. The current " flowed 

 in one channel, alternately backwards and forwards, through the main stem and lateral 

 branches of a plume, and through the root, as far as the opacity admitted of its being 

 traced ; sometimes it was seen to continue into the cells. The stream was throughout in 

 one direction at one time ; it might be compared to the running of sand in an hour-glass, 

 and was sometimes so rapid in mid-tide that the particles were hardly distinguishable ; 

 but it became much slower when near the change. Sometimes it returned almost without 

 a pause ; but at other times it was quiet for awhile, or the particles took a confused 

 whirling motion for a few seconds ; the current afterwards appearing to set the stronger 

 for the suspension." " Five ebbs and five flows occupied fifteen minutes and a half; the 

 same average time being spent in the ebb as in the flow." Lister states that the vibrating 

 motion of the internal axial fluids were first observed by Cavolini, and are described in 

 his Memorie per servire alia Storia de 1 Polipi Marini, published at Naples, in 1785. 



6 



