60 ILLINOIS BIOLOGICAL MONOGRAPHS [254 



They are very rigid and act as a series of jaws. In structure and general mode 

 of action, this resembles the bulbous pharjmx of the Prorh)aichidae, where the 

 rosette ceUs number only four. The general shape of the whole rosette is 

 changed very quickly and the complexity of its action makes possible a wide 

 range of feeding habits. 



The intestine into which the pharynx opens broadly is comparatively thin- 

 walled. This bounding wall which is very transparent and elastic is made up 

 of one or two layers of cells which act as an absorptive surface and are subject 

 to an enormous amount of variation in size. When there is little food material 

 either in the main portion of the intestine or in some part of it, the wall draws 

 in so that the cavity is entirely obliterated. This is brought about by the 

 pressure of the surrounding parenchyma as well as by the compressibility of 

 the lining cells. On the other hand, when there is need for greater amount 

 of space in the aUmentary tract, the wall is crowded out in all directions, 

 portions of it compressing the muscle strands and other organs. The anterior 

 end often reaches as far forward as the very tip of the head, pu;shing its way 

 dorsal to the brain ganglion and the eyes. 



The digestive processes are carried on by means of these simple cells in 

 the lining wall. In some manner the food material is reduced to very minute 

 clear globules which are often ejected as the result of slight pressure or other 

 stimulus. These globules are absorbed, converted during the process into 

 an oil, and stored as droplets in the outer layers of the intestinal wall. These 

 droplets are extremely variable in size and color. Those found in an individual 

 at any one time will range from many which are just visible to some one-third 

 as large as the pharynx rosette. The color is always very clear, a brilliant 

 yellow-pink or some shade much darker. Generally, all the globules in a single 

 specimen are the same tint but there is the greatest difference among several 

 individuals taken from the same very small aquarium jar. 



These rather striking details in the structure of the digestive system are 

 characteristic of this family and sharply mark it from all others. 



NERVOUS SYSTEM 



The nervous system contains fewer cells and a smaller bulk of protoplasm 

 than any other part of the whole structure. The several tissues and organs 

 are so arranged that they are very easily reached by means of a rather scant 

 supply of nerve material. The kinds of stimuH received are few, as a conse- 

 quence the receptive apparatus is not at all complex and the problems of correl- 

 ation and response are solved without great difficulty. A rather high degree 

 of sensitiveness in certain directions and the possibihty of a precise response 

 seem to have been achieved with a minimum of anatomical detail. As a result 

 of such primitive plan, the nerve mass is not sharply marked off into separate 

 divisions but appears as a very homogeneous, almost undifferentiated whole. 



The main or central part of the nerve tissue is the brain ganghon which 

 is situated near the anterior tip of the head, on a line with the widest place. 



