UNSEGMENTED WORMS 1 93 



species; of worms are found not infrequently as intestinal parasites in cats, dogs, or 

 other animals dissected in the laboratory. The general form, the method of 

 attachment to the host, the progressive development of the proglottides or "seg- 

 ments," and the difference between these segments and those of the earth-worm 

 should be noted. Permanent whole mounts of a mature proglottis may be made, 

 showing the embryos in the uterus. Demonstrations of the structure of the pro- 

 glottis may be given by properly prepared transverse sections, if the equipment 

 and time allow. 



An hour's work may profitably be devoted to the study of some one or more of 

 the common Rotifers, which may be found in water taken from the stagnant pools 

 in which there is much decaying matter. They are microscopic animals and are to 

 be recognized by the possession of discs at the anterior end, which present the 

 appearance of rotating wheels because of a rhythmic action of the cUia. Make 

 sketches showing the change of shape which the animal undergoes. How is the 

 change effected? How is locomotion accomplished? What evidences have you 

 of its ability to receive stimuli and to respond to them? How does it get food? 

 Can you trace the digestive tract in the body of the animal? Notice the contract- 

 ing object just back of the mouth. What conclusions do you reach as to its func- 

 tion? Give your evidences. Verify by consulting some textbook. Can you 

 prove from what you see that this is not a single-celled animal like Stentor? These 

 specimens should not be considered as closely typical of the whole group of Rotifers, 

 since there is very great variety of form among them. 



Planarians often appear in the laboratory in water containing an abundance 

 of decomposing organic matter, taken from ponds and foul streams. The most 

 important points to be noticed are their general form, the method of locomotion, 

 sensitiveness to stimuli, and life habits. Asexual reproduction by fission is fre- 

 quent among them. 



The Polyzoa occur as tufts of many minute animals in colonies attached to 

 objects in the water. Plumatella is a rather common fresh- water form and makes 

 a beautiful demonstration to illustrate the ordinary physiological processes, as 

 motion, feeding, the action of the digestive tract in churning the food, sensitive- 

 ness to stimulus and the like. Schools near the sea-shore will find an abundance 

 of marine material for the comparison of the colonial forms of different species 

 of Polyzoa, since they are more common in salt than in fresh water. 



235. Classification and Description. Phylum Platyhelminthes (Flat-worms). — 

 In the worms of this phylum the body is flattened or compressed in a dorso- 

 ventral direction, and from this fact the name is given. They are soft-bodied 

 animals without any true skeleton. There is no body cavity and no true blood- 

 vascular system. The space which would be given to such structures is filled with 

 a solid mesoderm. Through this body-mass run the minute tubes of the excretory 

 or water- vascular system (Fig. 88, ex.), often terminating internally in special 

 cells {flame cells, Fig. 90). These tubes have external pores. By means of this 

 system of organs waste products, probably of a nitrogenous nature, are eliminated 

 from the tissues. The digestive tract may be wholly wanting as in the Cestodes, 

 or be a simple or forked sac, or a central sac with lateral branches. When present, 

 it is blind, i.e., has only the oral opening. In the more complicated types of 

 digestive tract the much-branched sac serves the function of carrying the digested 

 food to all parts of the body. Many of these forms are parasitic and in conse- 

 13 



