THE EARTHWORM 



265 



divided into annelids, or segmented worms, plathyhelminthes or flat 

 worms (Gr. platy=flat-j-helminthes= worms) ; and nemathelminthes 

 or thread-worms (Gr. nema thread-]- helminthes=:worms). 



The important external characteristic in the annelids is, then, a re- 

 gional differentiation. That is, the forming of separate segments or 

 regions externally, and a separation and segmentation of many internal 



structures. Metamerism is com- 

 mon in all higher forms of organ- 

 isms except the soft-bodied animals 

 such as the Molluscs and the spiny- 

 skinned Echinoderms. In Man this 

 metamerism is distinctly shown in 

 the separate segments of the spinal 

 column. 



There are many differentiations 

 in various regions of the earth- 

 worm's body. For example, the 

 anterior end is sensitive to touch 

 and light to a much greater degree 

 than the middle and posterior por- 

 tions. On the eighth, ninth, four- 

 teenth and fifteenth segments there 

 are openings of the reproductive 

 system, while from the twenty- 

 eighth to the thirty-seventh seg- 

 ments a broad band surrounds the 

 dorsal and lateral portions of the 

 worm called a clitellurn, the func- 

 tion of which will be explained un- 

 der Reproduction. 



There are from 140 to 180 seg- 

 ments in the earthworm. All of the 

 differentiation just mentioned oc- 

 curs toward the anterior end of the worm. We therefore say the earth- 

 worm has an anterior-posterior differentiation. 



As the earthworm will always place itself in a definite position 

 when crawling along that is, will "right" itself if it be turned about, 

 we speak of that portion toward the surface on which it moves as the 

 ventral surface and the surface away from this as the dorsal. If an 

 animal thus rights itself there must be a difference between the ventral 

 and dorsal surfaces. This difference is spoken of as a dorso-ventral dif- 

 ferentiation or dorsiventrality. 



The ventral surface will be found to be more flattened than the dor- 

 sal, while many little whitish glands are present toward the anterior end. 

 On the ventral surface are also found the mouth, anus, reproductive, 



Fig. 165. 



Latero-ventral view of Lumbricus ter- 

 restris, slightly smaller than life-size. (From 

 Hatschek and Cori). 



1. Prostomium. 2. Mouth. 3. Anus. 4. 

 Opening of oviduct. 5. Opening of vas de- 

 ferens. 6. Genital chaetae. 7. Lateral and 

 ventral pairs of chaetae. 



XV, XXXII, and XXXVII are the 15th, 

 32nd, and 37th segments. The 32nd to the 37th 

 form the clitellum. (After Latter). 



