164 METAZOAN PHYLA 



Although both male and female germ cells are produced in the same . 

 individual, cross-fertilization rather than self-fertihzation occurs. The 

 egg cells received by the atrium from the vagina are passed into the 

 uterus, where they are fertiUzed. When cross-fertiUzation takes place, 

 the cirrus of one animal is protruded and inserted through the genital 

 atrium into the uterus of the other. In this manner sperm cells are 

 transferred from the animal which takes the part of a male to the uterus 

 of the other which takes the part of a female. Every egg cell is sur- 

 rounded by a large number of nurse cells, or yolk cells, each of which 

 contributes its store of nourishment to the egg cell to which it becomes 

 attached. One or more egg cells, with the yolk cells and the yolk, are 

 then enveloped in a shell, sometimes called a cocoon, and deposited on 

 stones or on vegetation in the water. The egg cell divides into blasto- 

 meres which increase in number and finally form a blastoderm sur- 

 rounding a central cavity filled with yolk. Into this cavity are budded 

 off cells which arrange themselves in such a way as to form a sheet of 

 entoderm surrounding an innermost cavity which becomes connected 

 with the outside and forms the gastrovascular cavity. Between the 

 entoderm and ectoderm cells derived from them multiply and form the 

 mesoderm. 



191. Behavior.— A fresh-water planarian moves with no apparent 

 effort, gliding over a surface and adjusting itself easily to every irreg- 

 ularity, this being made possible by the softness of its body. As it 

 progresses it raises its head and turns it from side to side as if feeling 

 its way. The cilia are locomotor structures, but they are so minute 

 as to be invisible to the eye and thus the worm seems to shde over the 

 surface on which it moves. The motion is rhythmic, waves of move- 

 ment passing backward from the head. The ciha beat in a mass of 

 shmy mucus secreted by glands on the under surface of the body, which 

 forms a track for the animal, laid down as it progresses. 



The animals respond to the same stimuh which are effective in 

 other lower organisms— that is, to contact, to temperature, to fight, 

 to chemicals, and to water currents. They find their food by both 

 chemical and contact stimuh. The chemical attraction of the juices 

 of the food seems to bring them to it. When found, the food is held 

 between the head and the substratum and compressed by the body 

 before the animal moves forward to bring the proboscis in con- 

 tact with it. A planarian reacts negatively to a variety of substances 

 in strong solutions and positively in weak solutions, the effective strength 

 varying with the substance. The eyespots seem to be fight-perceiving, 

 though of course the animal possesses no vision. Planarians alternate 

 periods of activity with those of rest and are more active at night than 

 during the day. 



