CCELENTERATA. 175 



depends on the law of budding or non-sexual reproduction 

 of the polyps, and the activity shown by the individual in 

 secreting. In some cases single polyps produce a skeleton (cup- 

 corals). The coral reefs of tropical seas are illustrations of 

 the power of corals to form and excrete carbonate of lime. 

 Much of the lime-stone of the earth's crust shows that corals 

 assisted in its formation. 



222. Sensation. The nerve cells may be scattered diffusely 

 over the surface of the body with a mesh- work of fibrils to 

 connect them with the muscular and nettle cells and with each 

 other, as in Hydra. In some other polyp-forms there is more 

 differentiation of cells and fibres, but the elements are still 

 scattered. In the more active types there is a collection of 

 the cells either as a connected ring, or in groups, in the ten- 

 tacle-bearing rim of the animal. Associated with this collec- 

 tion 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 " olfac- 

 tory " pits. The tactile sense is undoubtedly present and the 

 chemical sense (taste or smell), although no special organs 

 are apparent. Otocysts (see 108) are found in the cteno- 

 phores and in some medusae, land apparently function chiefly as 

 organs of equilibration. 



223. Reproduction and Development. The occurrence 

 of both sexual and asexual methods of reproduction has al- 

 ready been mentioned (218). It is by the latter method that 

 colonies are normally produced and a given locality well occu- 

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

 is effected, and new regions are occupied. The ova and sper- 

 matozoa develop in special gonads (ovaries or testes) derived 

 either from the ectoderm or 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 



