﻿April, 1857.] 



ELLIOTT SOCIETY. 



155 



portion of the polyp's body also is dilated into a kind of disk, the 

 circumference of which bears the lower whorl of tentacuia. 

 These latter, like the corresponding ones in Pennaria, are ca- 

 pable of only very limited contraction. The horny polypidom 

 though sometimes branched is never branched so as to produce a 

 regular pattern as in the case of Pennaria. 



This group includes among its larvae the largest of known hy- 

 droid polyps. The stems of Tubularia indivisa (?) sometimes 

 attain a height of more than a foot. (See Dalyell's Rare and Re- 

 markable Animals of Scotland, vol. t, p. 3.) The only approach 

 to them in size is made by the communities of the genus Physalia, 

 (which, however, I look upon as communities of medusae not of hy- 

 dras,) whose air-bladder, which must be looked upon as the base of a 

 community, is sometimes nearly a foot in length, with the extended 

 tentaculiform individuals measuring nearly three feet in length. 



TUBULARIA. Linneeus. 



The genus Tubularia contains those fixed hydroids whose pipe- 

 like horny stem is surmounted by a polyp broad below and taper- 

 ing above, and encircled by two whorls of simple tentacuia, 

 between the upper and lower of which the medusa-buds spring 

 from the smooth untentaculated space. The polyp-stem is contin- 

 ued below in a creeping intertwining root. 



The digestive cavity of the polyp does not extend into the 

 expanded base, except during the young and locomotive stage of 

 the larva. The tentacuia in both whorls are generally numerous, 

 and not clavate at their extremities. The medusa-buds are some- 

 times each separately attached by a short pedicle to the sides of 

 the polyp; sometimes hang in several grape-like clusters of great 

 beauty among the tentacuia of the lower whorl. 



There are at least two groups in this genus which may be dis- 

 tinguished by their Medusa-forms. In some, as in T. calamaris, 

 and T. Dumortierii, whose development has been traced by Van 

 Beneden, the Medusa has four tentacuia and becomes free. In 

 T. indivisa on the contrary, as observed by Dalyell; T. coronata, 

 by Van Beneden; in a species of the Mediterranean observed by 

 Kolliker, and the analogous species about to be described from 

 Charleston Harbor, the Medusae want tentacuia, do not become free, 

 but hanging in clusters, nurse within their bell-cavities, round 

 embryos which there become tentaculated and at last escaping 

 thrust themselves out by means of their tentacuia, and afterwards 



