CHAPTER XI 

 THE ANATOMY AND PHYSIOLOGY OF ANNELIDS 



General Form of the Body. — The annelids or ringed worms 

 (Fig. 159) are of especial interest Ijecause they seem to be 

 the ancestors of all the higher animals having a l^ody composed 

 of rings or segments. Such are all C-rustacea, Tracheata 

 (including insects), and "\'ertebrates. The origin of the 

 ringed condition of the body is uncertain ; the lower worms do 

 not have it, so it must have arisen from the unsegmented con- 

 dition. We can help to understand how segmentation of the 

 body arose l)y inciuiring as to its present meaning. The rings 

 are more distinct when the worm contracts, less so when it 

 expands in any part ; consequently the rings are due to the 

 muscles of the body wall. Now in the most perfect annelids 

 the cavity of the Ijody is divided by partitions into a series 

 of compartments, reseml)Iing the water-tight compartments of 

 an ocean steamer (Fig. 161). The muscles of the body-wall 

 rim in each compartment from one partition to the next, so 

 when they contract, swelling, like all muscles, chiefly in the 

 middle and little at the attached ends, they cause the wavy 

 outline of the body that we see in the contracted worm. From 

 one constriction (partition) to the next is one segment. The 

 reason why the bod.y muscles run in this way is to permit the 

 worm to writhe — a mode of loconrotion very useful in moving 

 through mud or even water. The writhing, or S-shaped move- 

 ment of the body (Fig. 182) necessitates, however, contract- 

 M 161 



