WORMS. 



179 



mouth. Aside from the digestive cavity, the body is solid 

 throughout, there being no such body-cavity as occurs 

 in the higher worms, the echinoderms and the vertebrates. 



The nervous system consists of a centre, 

 or 'brain/ always in the dorsal front por- 

 tion of the bodv. from which nerve-cords 



\j / 



run to various parts, there 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 many 

 species, especially in the non-parasitic 

 groups. In these a sec- 

 ond 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 be- 

 fore the division is com- 



FIG. 23.-Process of di- FIG. 24.-A Tur- plete, SO that W6 Can 



have a chain of four or 



even eight animals con- 



___^__ nected together, and all 



the result of division of 



a single parent (fig. 23). Besides this reproduction by 

 division, reproduction by means of eggs occurs. The Pla- 

 thelminthes are divided into three orders- -Turbellaria, 

 Trematoda, and Cestoda. 



"I 



:nn 



m- 



m. 



vision in Microstomum. 

 After Graff. m, m~, 

 mouths of successive 

 generation; I-V, suc- 

 cessive planes of divi- 

 sion. 



