CCELENTERATA. 25 



(2) Sexual Reproduction. The wall of the ripe ovary bursts, 

 and part of the surface of the now spherical ovum is exposed. 

 By the rupture of the spermary at its apex, the sperms are 

 liberated, and by the action of their tails move rapidly head first 

 through the water. Impregnation or fertilization of the ovum now 

 takes place, that is, a sperm (derived from the same or a different 

 individual) fuses with it to form an oosperm. This union of a 

 small active male cell with a relatively large and passive female 

 one, is the essential part of sexual reproduction. 



4. Contractility. Amoeboid and ciliary movements are seen 

 in the endoderm. Besides these, Hydra is capable of movements 

 on a larger scale. The body and tentacles can be extended and 

 contracted, their appearance varying very much in consequence, 

 and curvings, very complex in the case of the tentacles, can also 

 be effected. These movements may lead to locomotion, either slow 

 by the gradual shifting of the foot, or more rapid by the alternate 

 sucker-like attachment of the foot and the mouth. Hydra has 

 also been seen creeping along on its tentacles, the body being 

 held almost vertical. It can also float passively near the surface 

 of the water, mouth downwards. 



The retraction of the body subserves protection, whilst the ten- 

 tacular movements are mainly concerned with the procuring of 

 food. All these movements are the result of longitudinal and 

 tranverse contractions, separate or variously combined. The tails 

 of the large ectoderm cells are arranged longitudinally, and by 

 their shortening or contraction furnish the retracting element. 

 If the endoderm-cell really possesses traversely-arranged tails, 

 their contraction would give the necessary extending element. 

 If this is not the case, the conical parts of the tailed-cells could 

 effect the same purpose, by contracting traversely, the result being 

 a corresponding increase in length. 



Discharge of a nematocyst is caused by the contraction of the 

 cnidoblast. By this means the contained fluid is put under pres- 

 sure, and everts first the neck, and then the hollow filament. 

 There is no contrivance for effecting the reverse process, and 

 nematocysts, when once used, are therefore discarded. 



5. Irritability and Spontaneity. Hydra is extremely sensitive 

 to external stimuli, and this is especially well seen in the move- 

 ments of retraction and in the discharge of nematocysts. The 

 cnidocils are sensory, that is to say, specially modified for the 



