DIVISIONS OF THE HYDROZOA. 63 
naked eye, though it can only be satisfactorily examined by the 
help of the microscope. When uncontracted, the body of the 
Hydra is in the form of a cylindrical tube (fig. 16, a, 4), com- 
posed of the two fundamental layers, the ectoderm and en- 
doderm, of which the former contains many thread-cells, and 
is likewise furnished with numerous green granules, stated to 
be identical with “chlorophyll,” or the green colouring-matter 
of plants. At the base or proximal extremity of the cylindrical 
body is a kind of disc-shaped sucker or hydrorhiza, by means 
of which the animal can attach itself at will to any foreign body. 
Its favourite position appears to be that of hanging head-down- 
wards, suspended from the stem of some water-plant. Itis not, 
however, permanently fixed, but it can detach itself and change 
\ 
Vig, 16.—Morphology of Hydrozoa. a Diagrammatic section of Hydra: the dark 
line is the ectoderm, the fine line and clear space adjacent indicate the endoderm. 
6 Hydra viridis, showing a single ovum contained in the body-wall near the 
proximal extremity, and two conical elevations containing sperm-cells near the 
bases of the tentacles. c Hydra vulgaris, with an undetached bud—enlarged. 
@ Thread-cell of the Hydra, greatly magnified. 
its place at will. At the opposite or distal extremity of the body 
is placed the aperture of the mouth, surrounded by a circle of 
from five to fifteen small tubular filaments, which are termed 
the “tentacles” (fig. 11, 4). Each tentacle consists of a tubular 
prolongation of both ectoderm and endoderm, and encloses a 
canal which opens at its base into the general cavity of the body. 
The ectoderm of the tentacles is richly furnished with thread- 
cells, and they are also well supplied with muscular fibres. 
They exhibit the most extraordinary contractility, being capable 
of retraction till they appear as nothing more than so many little 
