Flatworms 171 



taken for large ciliate protozoans, if viewed only with 

 the unaided eye ; but under the microscope the alimen- 

 tary canal and other internal organs are at once 

 apparent. They are multicellular and have little like- 

 ness to any infusoria, save in the ciliated exterior. 

 Most members of the group are flattened, as the com- 

 mon name suggests, but a few are cylindric, or even 

 filiform. A few are inclined to depart from shelter and 

 to swim in the open water, especially at time of abund- 

 ance. Kofoid ('08) found them in the channel waters 

 of the Illinois River in average numbers above 100 per 

 cubic meter, with a maximum record of 19250 per cubic 

 meter. 



The large flatworms resemble leeches somewhat in 

 form of body, but they have more of a head outlined 

 at the anterior end. They lack the segmentation of 

 body and the attachment discs of leeches, and their 

 mode of locomotion is so very different they are readily 

 distinguished. They do not travel by loopings of the 

 body as do leeches, but they glide along steadily, pro- 

 pelled by invisible cilia. 



The most familiar flatworms are the planarians: 

 soft and innocuous -looking little carnivores, having 

 the mouth opening near the midventral surface of the 

 body, and the food-cavity spreading through the body 

 in three complexly ramifying branches. They are 

 often brightly colored, mottled white, or brick red, or 

 plumbeous, and they have a way of changing color with 

 every full meal; for the branched alimentary canal 

 fills, and the color of the food glows through the skin 

 in the more transparent species. The eggs of planarians 

 are often found in abundance on stones in streams in 

 late summer. They are inclosed in little brownish 

 capsules, of the size and appearance of mustard seeds, 

 and each capsule is raised on a short stalk from the 

 surface of the stone. Increase is also by automatic 



