HYDRA AND CCELENTERATES. 131 
actively about and finally coalesces with an ovum from 
another hydra. This process is called cross-fertiliza- 
tion. Animals which produce in the same individual 
both ova and sperms are said to be hermaphrodite. 
We can see that this cross-fertilization does not differ 
materially from that already described as occurring in 
the white clover. The eggs after fertilization are called 
odsperms. These odsperms develop into hydras like 
the parents. In the autumn, eggs are produced with 
Fic. 111.—Forms of Hydra (magnified). A, half retracted; 2, fully 
retracted; C, extended; /, foot; ovm., ovum; 4, bud; ¢s., sperm-cells; 
h.s., hypostome; ¢, tentacles. 
thick, hard shells to withstand the cold and thus pre- 
serve the species until spring. 
Discovery. The hydra has no nervous system, 
though a few nerve-cells are developed. It is sensitive 
to light and extremely so to touch. 
Movement. The hydra moves without muscles. It 
can move by pushing its sucking-disk or foot along the 
leaf to which it is attached or it can march by somer- 
saults to its destination. The tentacles seem to move 
at will. 
The Hydractinia.. Division of Labor. A salt- 
water animal which resembles the hydra is Hydractinza, 
