184 ZOOLOGY 



in the tentacle-bearing rim of the animal. Associated with this 

 collection of the nervous material into a kind of nervous centre, 

 there are often special areas of sensory epithelium, or sense 

 organs, developed from the ectoderm. It is not wholly clear 

 what kinds of stimuli they are suited to receive although they 

 are designated as "eye spots," or as "auditory" or "olfactory" 

 pits. Otocysts (see §111) are found in the Ctenophores and in 

 some medusae, and apparently function chiefly as organs of 

 equilibration or perception of disturbances in the water. 



Experiments show that coelenterates are sensitive to dif- 

 ferences in intensity of light, to mechanical stimuli, to tempera- 

 ture, to gravity, and to chemical stimuli. The responses may 

 be local, as when a Hydra withdraws its tentacle upon a light 

 touch or uses all its tentacles and the region about the mouth 

 when stimulated mechanically and chemically by its prey; or 

 general, as when it contracts body and tentacles completely 

 upon vigorous mechanical stimulation. 



In addition to such responses to external stimtili there are 

 in Hydra bendings of the body, wavings of the tentacles, and 

 even contraction and extensions of the whole body that suggest 

 "seeking" movements. Hungry specimens are more active 

 than weU-fed ones. All of this shows that even in the simplest 

 representatives of the group there is effective nervous and 

 muscular coordination. 



228. Reproduction and Development.^The occurrence of 

 both sexual and asexual methods of reproduction has already 

 been mentioned (§223). It is by non-sexual budding that 

 colonies are normally built up and a given locality well occupied 

 by the species. By means of the sexual method dispersion is 

 efifected, and new regions are occupied. The ova and sper- 

 matozoa develop in special gonads (ovaries or testes) derived 

 in some species from the ectoderm, in others from the entoderm. 

 The sexual cells usually escape into the gastro-vascular cavity 

 and reach the outside by way of the mouth. As a rule the sexes 

 occur in separate individuals. After fertilization cleavage is 

 total but soiiietimes not equal. A blastula is formed which is 

 often converted into a peculiar, free-swimming, ciliated larva 



