CHAPTER X 



PHYLUM ANNELIDA 



THE name Annelida (Lat. anmilus, a ring) means ringed, and 

 refers to the fact that the bodies of the creatures grouped under 

 this name are built up of a series of parts more or less resembling 

 each other placed one behind another. This division of the 

 body into more or less similar parts is called segmentation; 

 each part is called a segment (or somite), and the animal is 

 said to be segmented. Like the symmetry, the segmentation 

 may be merely external or may affect both the exterior and 

 a greater or less number of the internal organs. 



Segmentation. " . PIT 



sometimes, however, as in the case ot the longer 

 half of an earthworm's body, the segmentation affects all the 

 organs, and the likeness of one segment to another is so great that 

 it would be impossible to say what part of the body any given 

 isolated segment was taken from. More often, however, one or 

 another of the organs of the body differs in shape or size in 

 successive segments, and this is the case with the internal organs of 

 the first twenty segments of the earthworm's body, so that if these 

 segments were all separated it would not be very difficult to place 

 them together in their natural order. 



If we take an earthworm and kill it by placing it in alcohol for 

 The a few minutes and examine it carefully, we shall see 



Earthworm. that the body is composed of some 150 rings, each 

 Matures! ^ which corresponds with a segment. The rings are 



separated from one another by slight grooves. At 

 each end of the body there is an opening, the mouth (2, Fig. 59) 

 in front and the anus (3, Fig. 59) behind. Besides these, two 

 slit-like pores with rather swollen lips, situated on the under sur- 

 face of the fifteenth segment (5, Fig. 59), may be seen. These 

 are the pores through which spermatozoa are discharged, and 



