WORMS. 



243 



being usually two long cords which run backwards in a 

 nearly parallel direction. Eyes may be 

 present on the dorsal surface near the 

 brain. 



The capacity of reproduction by di- 

 vision is very well developed in these 

 forms, especially in the non-parasitic 

 groups. In these a second mouth will 

 appear at about the middle of the body, 

 then the body will constrict in front of 

 the new mouth, and finally will divide 

 into two worms. Not infrequently a 

 new mouth will appear in each of the 

 halves before the division is complete, so 

 that we can have a chain of four or even 

 eight animals connected together, and 

 all the result of division of a single pa- 

 rent. Besides this reproduction by di- 

 vision, reproduction by means of eggs 

 occurs. The Plathelminthes are divided 

 into three orders Turbellaria, Trema- 

 toda, and Cestoda. 



OKDEK I. TUKBELLAKIA. 



Fiq. 114. Process of 

 division in Microsto- 

 nnim. After Graff, wi, 



cessive 



planes of division. 



These are small free-living forms 

 which occur in fresh or salt water, and 

 occasionally in moist earth. They are 

 common in our ponds and streams, 

 crawling over the bottoms or upon submerged sticks and 

 stones. They have a mouth and digestive tract. 



OKDER II. TREMATODES. 



Like the last, these have mouth and digestive tract, but 

 they differ in being parasitic on or in other animals, and in 



