I9I5.] J. Rrrcwie : Hydroids of the Indian Museum. 557 
majority or all of them have the power of slight movement, and it 
is possible that they may be able even to withdraw from the mud 
and creep along the bottom. In any case, as Hartlaub has 
pointed out! these forms, both in their systematic affinities and 
the in their habit, present a well-defined half-way house between 
permanently fixed species and those which are able to leave their 
attachment and move freely on the substratum or in the sea. 
Amongst such temporarily creeping or floating forms we have 
the freshwater Hydra, andits relatives Protohydra and Polypodium ; 
the Tubularid, Hypolytus; and Haleremita of uncertain relation- 
ship, but closely resembling the larval stage of Gonionemus. Here 
also I am inclined to include the Pennarid, Tvichorhiza, which, 
found by Russell (1906) on the tentacles of Corymorpha nutans, 
was apparently caught in the act of moving. General but not 
universally present characters which link these forms (with the 
exception of Trichorhiza) are the almost total absence of perisarc 
and the presence of a basal thickening of coenosare—the pedal disc. 
I have not included definitely recognized larval forms, but perhaps 
the floating stage of Acaulis ought to be mentioned here, since 
floating individuals bear well developed medusae buds and may be 
considered adult. 
In a slightly more advanced category of unattached hydroids 
are to be placed the pelagic forms Margelopsis or Nemopsts, which 
represent the detached buds of such forms as Tzarella (see Bedot, 
IQII, p. 211); the unique Microhydra and the metagenic form of the 
Trachymedusan, Liriope which “is a true hydra, although its free- 
swimming mode of life and its superficial aspect caused it to be 
mistaken formerly for a gonosome ”’ (Perkins, 1903, p. 752). 
In none of these groups of unattached Hydroids is to be 
found an exact parallel to our Indian brackish-water species, the 
adults of which are at one stage firmly attached, and at another 
are released from their attached portions in order to lead a tempo- 
rary free (? pelagic) existence. But, as we shall see in discussing 
the basal bulb, that structure links Annulella with the creeping 
type, especially common in the family Hydridae. 
TENTACLES AND CNIDOBLASTS. 
The arrangement of the cnidoblast batteries of the tentacles 
in well-defined projecting rings is characteristic of very few hy- 
droid stages. It is, however, moderately common in the medusoid 
generations, being exhibited in such well-known forms as Thamno- 
stylus dinema, or in the medusoids of Corymorpha nutans , Stauridium 
and Syncoryne eximia.? In the hydroid stage, so far as I know, it 
is confined to Trichorhiza brunnei, Russell 1906, Heterostephanus 
annulicornis (M. Sars 1859), Hypolytus peregrinus, Murbach 1899, 
Asyncoryne ryniensis, Warren 1908, and occurs to a limited extent 
! Hartlaub, 1902, p. 29. 
2 Compare particularly the representations of the last species as drawn by 
Allman, 1871, pl. v, figs. 3-4. 
