THE EARTHWORM 281 



There are also separations from the mesoderm forming nephroblasts 

 if they form nephridia, somatoblasts which form muscles, etc. 



The ectoderm turns in at both anterior and posterior ends, the for- 

 mer forming the mouth or stomodeum ( ), the latter 

 the anal opening or proctodeum ( ). 



The chlorogogen cells are formed from mesoderm, as are also the 

 blood-vessels, muscles, reproductive organs and seta sacs. The young 

 worm is now ready for an independent life, and it leaves the cocoon 

 after from two to three weeks. 



The following table will give a summary of the important tissues 

 derived from the various germ-layers : 



ENTODERM ECTODERM MESODERM 



Oesophagus, Outer Epithelium, Muscles, 



Crop, Nervous System, Coelomic Endothelium, 



Gizzard. Stomodeum, Chlorogogen Cells, 



Proctodeum, Calciferous Glands, 



Ends of Nephridia. Blood vessels, 

 Septa, 



Nephridia, functional parts, 

 Seta Sacs, 



Reproductive Organs. 

 BEHAVIOR 



As already seen, worms are apparently fond of having their bodies 

 in contact with solid objects as shown by their home-life. Moisture 

 causes a positive reaction if such moisture comes in direct contact with 

 the worm's body. This is well illustrated by placing the earthworm, 

 Allobophora foetida (the small manure worm), on a piece of dry filter 

 paper when it will not react, but as soon as moisture is applied it begins 

 to burrow, provided this moisture or liquid is taken from manure. 



Darwin supposed that the earthworm's ability to distinguish edible 

 from inedible food lay in the sense of contact. This would make contact 

 in the earthworm act as a sort of taste organ. Various chemicals which 

 cause a reaction may be due to this sort of secondary taste-ability. 



While there are no eyes, light causes the animal to react as shown 

 by its moving away from lighted areas though the manure worm will 

 respond positively to a very faint light. The preferable colors, when 

 very faint, are red, green and blue in the order given, though it does not 

 follow from this that the earthworm can distinguish 'colors ; its ability 

 consisting, in all probability, of "feeling" different rays of light as well 

 as different intensities. 



It has also been noted that if a previous stimulus is much stronger 

 than a succeeding one, the first will naturally continue to react and cause 

 either no reaction to a second or at least lessen such reaction. An exam- 

 ple of this is found when the animal is feeding or mating. Light which 



