230 



ELEMENTARY BIOLOGY 



269. Light tropisms. There are many simple animals that 

 are ordinarily phototropic in the positive sense, and there are 

 many that are negatively phototropic. In some cases, as in the 

 Euglena, the tropism can be reversed', that is, made to be the 

 opposite of what it was. An agitation of the water, an electric 

 shock, a change in the temperature, may reverse the sense 

 ^ of the phototropisms. This would 



}sf^ show that the response depends up- 



on the condition of the protoplasm. 



270. Mollusks and light. Com- 

 paratively few of the mollusca 

 (oysters, clams, scallops, etc.) have 

 special light organs. Most of the 

 common bivalves have a region 

 about the edge of the mantle that 

 is sensitive to light. In the scal- 

 lops there are definite eyespots at 

 the edge of the mantle. In the 

 snails, the squids, and the octopus 

 there are definite eyes, those of the 

 octopus resembling the eye of the 

 backboned animals in many ways. 



271. General sensitiveness. The 

 earthworm has no eyes, but the 



whole skin, and especially that near the front end of the body, 

 is sensitive to light. The worms will crawl away from the 

 source of light unless the illumination is very low. Thus they 

 keep out of sight during the day, and crawl to the openings of 

 their burrows at night. 



272. Compound eyes. Insects and other arthropoda com- 

 monly have compound eyes, and many of them have also sim- 

 ple eyes (Fig. 96). There are many nerve-cell endings in 

 each of the eyes, and as the lens projects a tiny image upon 

 these endings, there is formed a patchwork of varying lights 

 and shadows, some of the cells being highly illuminated, others 



FIG. 94. Eyespots in starfish 



The eyesfot at the end of each ray is 

 connected with the nervous system 

 of the animal and is more sensitive 

 to light than the rest of the body 

 surface. In this group of animals 

 (Echinodermata, or spiny-skinned) 

 there is a central nervous system, 

 so that there are true reflexes 



