DIVISIONS OF THE HYDROZOA. 65 
without any apparent inconvenience or interference with its 
health. 
Reproduction in the Hydra is effected non-sexually by gem- 
mation, and sexually by the production of ova and sperm-cells; 
the former process being followed in summer and the latter in 
autumn, few individuals appearing to survive the winter. In 
the first or non-sexual method, the Hydra throws out one or 
more buds, usually from near the fixed or proximal extremity 
(fig. 16, c). These buds at first consist simply of a tubular pro- 
longation of the ectoderm and endoderm, enclosing a cavity 
which communicates with the general cavity of the body. A 
new mouth and tentacles are soon developed at the distal end 
of this bud, and after a longer or shorter period the new Hydra, 
thus produced, is detached to lead an independent life. Each 
Hydra can produce many such buds during the summer season, 
and the liberated buds can also repeat the same process, so that 
in this way reproduction is rapidly carried on. In the second 
or sexual method of reproduction, ova and sperm-cells are pro- 
duced towards the winter in external processes of the body-wall. 
The spermatozoa are developed in little ‘conical elevations, 
which are produced near the bases of the tentacles ; and the ova 
are formed in much larger elevations, of which there is ordi- 
narily but one, placed nearer to the fixed or proximal extremity 
of the animal (fig. 16, 6). When mature, the ovum is fertilised 
by the sperm-cells, both being set free into the water by the 
rupture of the body-wall. The embryo “Hydra is at first cov- 
ered with vibrating cilia, and swims freely about, until it meets 
with a suitable locality. It then fixes itself by one extremity, 
the cilia drop off, and a mouth and tentacles are developed at 
the distal end of the body. 
ORDER II. CorynipA.—In the second order of the Hydroid 
zoophytes, known as the Corynida or Tubularida, we have a 
number of organisms which in their essential structure are 
closely related to the Hydra, but which differ considerably in 
the nature of the reproductive process. All of them are marine, 
with the single exception of the genus Cordylophora, which in- 
habits fresh water. Some of the members of the order are 
simple, consisting of no more than a single polypite. In these 
cases there is an exceedingly close approach to the structure of 
the common Hydra, but the polypite is permanently fixed with- 
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