68 



INVERTEBRATE ANIMALS. 



the help of the microscope. When uncontracted, the body of 

 the Hydra is in the form of a cylindrical tube (Fig. 13, a, 5), 

 composed of the two fundamental layers, the ectoderm and 

 endoderm, 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 coloring- 

 matter of plants. At the base of the cylindrical body is a 

 kind of disk-shaped sucker, by means of which the animal can 

 attach itself at will to any foreign body. Its favorite position 

 appears to be that of hanging head-downward, suspended 

 from the stem of some water-plant. It is not, however, per- 

 manently fixed, but it can detach itself and change its place 

 at will. At the opposite 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 " tenta- 



FIG. 13. 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 lower extremity, 

 and two conical elevations containing sperm-cells near the bases of the tentacles. 

 c Hydra vulgaris, with an undetached bud enlarged ; d Thread-cell of the Hydra, 

 greatly magnified. 



cles " (Fig. 13, b). Each tentacle consists of a tubular prolon- 

 gation 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 capa- 

 ble of retraction till they appear as nothing more than so 

 many little warts or tubercles, and of being extended to a 



