HYDROIDEA. Q1 
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 1a, and is tipped with a row of oral tentacles immediately about 
the mouth opening. ‘The tentacles of the Tubulariz and Campanu- 
laride, are described as differing from those of the Sertularide and 
Hydree, in not being properly tubular organs. 
The stems and branches of these zoophytes are tubu- 
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 
Campanularide 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 Sertularide (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.” 
motion of the internal axial fluids were first observed by Cavolini, and are described in 
his Memorve per servire alla Storia de’ Polipi Marini, published at Naples, in 1785, 
6 
Lister states that the vibrating 
