ORGANIC EVOLUTION 159 



first appear as a circle of rounded knobs about the free end, 

 and later elongate. A mouth breaks through at the distal 

 end of the body, and finally, the base constricts itself off, 

 closing communication with the internal food cavity of the 

 parent, and develops a foot for independent attachment. 

 Then the little hydra drifts away to set up in business on its 

 own account. This is obviously an asexual process of 

 reproduction. 



The sexual process is not so often observable. The sexual 

 organs, when present appear as minute transparent swellings 

 on the surface of the body ; conical and situated above the 

 middle, if spermaries; low and broadly domeshaped and 

 situated nearer the foot, if ovaries. 



FIG. 101. Hydra, a, a sperm cell; b, an egg cell; c, diagram of a 

 longitudinal section of the body, bearing a lateral bud, ectoderm 

 white, endoderm black; d, a bit of the body wall showing tissue 

 .layers; e, dissociated cells with basal contractile processes; /, a 

 rudimental nerve cell. 



If a hydra be transferred to a slide or watchglass and 

 examined a little magnified, the two constituent layers of its 

 body wall may readily be made out; a transparent outer 

 ectoderm, and a darker inner endoderm. The endoderm 

 lines the food cavity, which is merely a blind sac having only 

 one opening, the mouth, through which food is taken in and 

 its indigestible residue is thrown out. The tentacles being 

 formed by outgrowth of both layers are hollow, each is a 

 similar smaller blind sac opening into the major cavity. 



