CHAPTER XXX 



FRESH-WATER PLANARIAN 



A TYPE OF THE PHYLUM PLATYHELMINTHES 



If the bottom of a spring-fed pool or the vegetation in it is carefully 

 examined, it is probable that there will be seen a number of soft, flat, 

 dark-colored worms gliding over surfaces with no apparent effort. These 

 are planarians. They may also be found in streams and in permanent 

 fresh-water ponds, in which the water is pure, cool, and clear. Some 

 species seem to prefer being in currents while Eye spot 

 others prefer quiet water. When at rest, pla- 

 narians tend to gather under stones lying on the 

 bottom or in other places where the light is ^^^ 

 not bright. When abundant they may be 

 attracted in great numbers to pieces of meat 

 deposited in the water along the shore. 



185. Structure. — A planarian (Fig. 71) has a 

 body which is elongated, flattened dorsoventrally, 

 blunt at the anterior end, and tapering to a 

 point at the posterior end. The anterior end 

 may be relatively square or it may be triangular 

 with the apex pointing forward. This portion 

 of the body is recognized as a head because it 

 goes ahead in locomotion and because it possesses 

 a nerv^e center, but it does not fulfill the ordinary 

 conception of a head since it does not contain a 

 mouth. There may be two lateral projections at 

 the base of the head which are termed ears, but sal view 

 these neither hear nor have any other function ^'"•f • ^'\ ^'f; ^i^">^-/'o"»/.'- 



•^ Zool., vol. 31.) B, the ani- 



of their own. mal turned to show features 



On the upper side of the head and near each "^ ^ii^ ^"^'^^''^l surface, x 6. 

 other in the median line of the body may be seen two eyespofs. On 

 the ventral side of the body as far back as the middle, or even farther, 

 is the mouth, through which may protrude the prohoscis. The surface 

 of the body is soft and is covered with ciha, which aid in locomotion. 



186. Internal Structure. — Careful examination shows a planarian 

 to possess many structures which have been found in none of the phyla 

 hitherto studied. The mesoderm is well developed, forms the greater 

 part of the mass of the body, and is composed of a meshwork of living 



167 



A B 



Fig. 71 . — P I ana r i a 

 maculala Leidy. .4, dor- 

 {Frorn Wood- 



