THE HYDROID MEDUSAE. 
347 
gives off a branch, which terminates in a sucker, and by- 
means of these suctorial appendages the Glavatella traverses 
the small rock-pools in which it finds a home, or mounting the 
tufts of sea-weed, seeks its food among their branches. A change 
in habit, mode of life, and range of distribution, accompanies 
the modification of structure, and the Glavatella appears to be 
separated from its kindred by a much wider gap than really 
exists. We have an interesting transitional form in Gladonema 
(fig. 7); in this case the bell is fully developed, and the 
creature is an active swimmer, but the tentacles (fig. 7 t ) are 
furnished with the sucker-bearing branch (fig. 7 s), and it thus 
enjoys the means of rapid locomotion and attachment at plea- 
sure, in combination. It does not appear to use its suckers as 
feet. In its adult state the tentacles of Gladonema bear several 
branches, and at least two of the suctorial appendages ; but, as 
I have lately had the opportunity of observing, in an early stage 
they are as simple as those of Glavatella , from which they only 
differ in the presence of a larger number of the groups of thread- 
cells (vide fig. 7 t). It should be remarked in passing that the 
free zooid of Glavatella , though of medusan type, makes a near 
approach to the polypite, and is an interesting link between 
the two principal elements of the Hydroid colony. 
Besides the ambulatory medusa, only one other deviation 
from the normal condition is known ; it occurs in the loco- 
motive sexual bud of the genus Dicoryne (fig. 9). In this 
extraordinary form the medusan structure seems to have 
vanished altogether. It consists of a closed sac covered with 
cilia within which the generative elements are developed, and 
bearing at one extremity two ciliated tentacles. It is clearly 
an intermediate form between the fixed buds, which in many 
hydroids discharge the generative function, and the ordinary 
medusa. In brief it is the equivalent of the central sac (fig. 
7, m) of the medusa, minus a swimming-bell and its appen- 
dages, made locomotive by the aid of cilia ; and the two ten- 
tacles may be regarded as rudiments of the contractile disc, 
the development of which has been, as it were, arrested in 
limine . 
But I must pass on to notice briefly the most important 
phase of the life of the medusa. It is its specific function to 
give origin, directly or indirectly, to the generative elements, 
and with the scattering of the seed its work is accomplished. 
In one section of the Hydroid medusae, the ova and spermatozoa 
are produced in the walls of the digestive cavity (fig. 7, m) ; 
in another, they originate in special sacs, developed on the 
course of the radiating canals (fig. 2, o, o). These sacs have 
long been regarded as the ovaries (or spermaries) ; but Allman 
has shown that in many cases at least, their structure is 
