26 A PLANARIAN. 



the proboscis. When drawn back it is enclosed in the pharyngeal 

 sheath. If you place bits of the meat of a crushed snail on the slide or 

 in the watch glass you may have a chance to see the proboscis extended 

 to grasp the food. 



The pharynx opens interiorly into a curious, much branched in- 

 testine divided into three main trunks. One of these runs forward in 

 the median line, the other two extend backward nearer the right and 

 left margins of the body. Trace them out in their coarser ramifications. 

 The light colored Planarians will be best for this study. Those from 

 the book gills of the King-crab are almost white. In their finer 

 ramifications the three main branches of the intestine reach all parts of 

 the body. 



By focusing sharply on certain parts of a specimen under heavy 

 cover-glass pressure other details can be at least faintly discerned. 

 Cilia may be brought out along the margins. Of what use are the 

 cilia ? A light colored area beneath the eye-spots indicates the location 

 of the main nerve ganglion or brain. From the brain light lines radiat- 

 ing forward represent sensory nerves and a faint streak down each 

 side of the body to the posterior end locates the longitudinal nerve 

 cords. The longitudinal nerves have many branches, transverse 

 nerves, some of which anastomose, forming commissures. 



The reproductive and excretory systems, though interesting, are too 

 difficult of demonstration for this exercise. Planarians do not have a 

 circulatory system or a fluid analogous to blood. Why are these not 

 necessary ? Respiration is carried on by the outer surface of the body. 



Exercise i. Draw one or more Planarians on a scale of six to ten, showing 

 all the detail you have been able to make out. The digestive and 

 nervous systems should be shown on separate drawings. 



Interesting experiments in regeneration may be conducted by the 

 student. Cut the animals into two or more pieces and keep them in 

 watch glasses of clean pond water. Keep records, in drawing, of the 

 original shape of the pieces and of the changes from day to day in the 

 form of each piece. 



