SURFACE FAUNA OF THE GULF STKEAM. 11 



sion of the base of the coenosarc may perhaps be considered as the first indi- 

 cation of the formation of a float. In this case this rudimentary pneumat- 

 opliore is attached to the ground or to a movable body-like Fiisus, etc., and 

 forms papilla^, between which arise the two dilTcrent kinds of polypites, the 

 sterile tentacular polypite and the rej^roducive polypite, giving rise to either 

 male or female colonies. This combination is exactly similar to tliat of Por- 

 pita, in which we have a chitinous float and tentacular and proliferous poly- 

 pites arranged in addition I'ound a central sterile polypite. 



The homologies we have attempted to trace between Porpita and the 

 Tubularian Hj'droids might perhaps be still further extended. It is well 

 known that in nearly all Tubularian.? the base of the coenosarc, by which 

 they are attached to the ground, extends either as filaments or rootlets over 

 a considerable space ; these filaments, or expansions of the chitinous tubes, 

 forming either a connected series of canals, moi-e or less complicated, as in 

 Clava, Cordylophora, Coryne, etc., or a net-work of canals as in Dicoryne, Bou- 

 gainvillia, Tubularia, Ilydractinia, Podocorj'ne, etc. ; or else such filamentary 

 processes as those of Corymorpha, in all of which there is a more or less active 

 circulation connected with that of the cavity of the Tubularian. In Cory- 

 morpha Ave find a series of longitudinal canals, more or less branching and 

 anastomosing, extending along the coenosarc. Let us now imagine this long 

 Corymorpha coenosarc reduced in length and at the same time flattened so 

 as to form a disk somewhat below the base of the tentacles, retaining its 

 peculiar pointed terminal basal extremity. We could thus have a free Hy- 

 droid differing but little from Velella and Porpita; that is, our Corymorpha 

 would be transformed to a Hj'droid, with a crown of marginal tentacles below 

 the chitinous disk, in which there are canals of the vascular system. Between 

 this row of marginal tentacles and the large central opening of the polypite 

 we find clusters of reproductive Medusoe. Imagine the same transformation 

 in a colony of Hydractinia, or of Podocoryne, in which we find the chitinous 

 disk already formed and traversed by a network of canals, and add to it a 

 central sterile polj-pite, and we have all the structural features of a modified 

 Porpita, namely, a disk, rows of tentacular polypites, next rows of repro- 

 ductive and feeding pol3^pites , while if we make the same comparison Avith 

 Hydractinia Ave add a third kind of appendage, the so-called spiral zoiJids. 



An examination of very young stages of Tubularians, such as, for instance, 

 the very early stages of Endendrium figured by Allman (PI. XIII, Figs. 

 14-16, Tubularian Hydroids), shoAv such a chitinous disk to be compared in 



