CCELE STERATA HYDROZOA. 135 



ible portions of the food are expelled through the 

 mouth. 



If a hydra is cut into two pieces, one portion will 

 develop a new trunk and base, the other will soon, 

 throw out new tentacles. In this way two perfect 

 animals may be formed from a single individual. If 

 it be cut into half a dozen or more pieces, each piece 

 will develop the wanting parts, and will grow into an 

 animal as perfect as the original. It may even be turned 

 inside out without seeming to suffer any inconvenience. 



193. Reproduction, in the hydra, is effected in two 

 ways by budding, and by ova. During the summer 

 season, little wart-like buds appear on the surface of the 

 body. These increase in size, and a mouth is formed, 

 which is soon surrounded by a circle of tentacles. In 

 its imperfect state, it receives nutriment from the 

 stomach of the parent; but when the tentacles are formed 

 it begins to procure food for itself, and the channel of 

 communication becomes closed up. When it has ar- 

 rived at maturity, it detaches itself, and becomes 

 independent. 



In autumn, ova and sperm cells are formed in little 

 elevations of the outer surface of the body wall. These 

 are deposited simultaneously in the water, and the 

 fertilized ova remain in the bottom of the pond, until 

 the next spring, when they are hatched. Very few 

 hydras, it is believed, live through the winter. The 

 embryo, at first, swims about by the action of the cilia 

 with which it is covered. It soon fixes itself by one 

 extremity, loses its cilia, and obtains a mouth and 

 tentacles. 



194. The Corynidse (Gr. korune, a club), or club-hydras, 

 are marine animals, with the exception of one genus 

 which inhabit fresh water. Some of them are simple, 

 and resemble the fresh- water hydra in structure; but 

 they are permanently attached to a solid object. The 

 greater number are compound, being united by a thread 

 of jelly-like flesh. These are generally provided with 



