82 ILLINOIS BIOLOGICAL MONOGRAPHS [276 



Of the species present in different localities, some occur much more widely 

 than others. Stenostoma leucops seems to be almost cosmopolitan and more 

 than any other is found in small ponds. Others, such as the genera of the 

 Microstomidae possessing simple anatomy, are also common. It seems evident 

 that the more primitive types adapt themselves easily to any surroundings 

 and also that they are not so responsive to environment in the way of variation. 

 On the other hand several members of the Typhloplanidae occur in many 

 places. They show a greater degree of anatomical variation and are thus able 

 to exist under diverse conditions. The two genera of the Prorhynchidae which 

 are Hsted here were found only at one place and then only as a few specimens. 

 The Dalyellids are another family not so widely scattered. So far, they have 

 appeared in only two or three places. The question of distribution is partially 

 one of chance and partially one of adaptability. The discussion above men- 

 tions only the general locaHties where the several families are found and the 

 most prominent reason for this occurrence. 



Any study of a number of families is likely to reveal certain characters of 

 comparative interest. Perhaps the most conspicuous detail of structure in 

 such a study of the Rhabdocoela is the shape which is at once significant and 

 important. Within the group there is a striking similarity of form even 

 among types differing essentially in anatomical particulars. Never long and 

 cylindrical like the higher worms, never very fiat like the related Polyclads, 

 these worms are intermediate between the two. Some species are almost 

 perfect in the spindle shape, large in the middle and tapering evenly toward 

 either end, others are very nearly cigar-shaped, the head being broadened and 

 slightly truncate. The most common variations are m the two extremities. 

 The head may be broad or pointed, rounded or sharply angled. The posterior 

 end often tapers very gradually making a long slender tail and often is broad- 

 ened to form a heavy square spatula. Generally the anteror end is somewhat 

 flat and the tail more or less cylindrical. Almost always the dorsal surface 

 is elevated thru the center. Most of the group swirn freely thru the open water 

 or wriggle their way thru dense masses of filamentous algae and a spindle- 

 shaped body is most easily propelled under such condition. The flatness of 

 some species such as the Prorhynchidae is clearly the best shape for those 

 types which habitually creep over smooth surfaces, as for instance the stems 

 and leaves of aquatic plants. Here they cling closely without presenting a 

 projecting mass by which they might be easily dislodged. Then, too, the 

 squarely truncated head does not cause any special inconvenience, as it might 

 in free-swinimuig forms. Another point which may have to do with the shape, 

 is the entire lack of locomotor organs or projecting parts of any sort, and since 

 the body is moved partly by muscular contraction and partly by the action 

 of the cilia, extreme smoothness is a great advantage. As a whole, the shape 

 is characteristic of the group and is closely related to the mode of life. 



The method of locomotion has been mentioned as either by means of 

 cilia or by muscular action, and the relative amount of development of each 



