266 



General Biology 



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

 or thread-worms (Gr. nema=thread+nelminthes— worms). 



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

 regional 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 clitellum, 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 occurs 

 toward the anterior end of the worm. We, therefore, say that 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 

 differentiation or dorsiventrality. 



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

 dorsal, while many little whitish glands are present toward the anterior 



Fig. 165. 



Lumbricus ter- 



(From 



Latero-ventral view of 

 restris, slightly smaller than life-size 

 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.) 



