248 PHYLUM ANNELIDA 



is usually necessary. Almost always plain on the 15th ring are the 

 two swollen lips of the male ducts, less distinct on the 14th are the 

 apertures of the oviducts through which the eggs pass, while on each 

 side, between segments 9 and 10, 10 and 11, are the openings of two 

 receptacula seminis or spermotheca; into which male elements from 

 another earthworm pass, and from which they again pass out to fertilise 

 the eggs of the earthworm when these are laid. Each segment con- 

 tains a pair of excretory tubes, which have minute ventral-lateral 

 apertures, while on the middle line of the back, between the rings, 

 there are minute pores, through which fluid from the body cavity 

 may exude on to the skin. 



Skin and bristles. — The thin cuticle is produced by 

 the cells which lie beneath, and is perforated by the 

 apertures previously mentioned. The epidermis clothing 

 the worm is a single layer of cells, of which most are 

 simply supporting or covering elements, while many are 

 slightly modified, as glandular or mucous cells, and as 

 nervous cells. As the latter are connected with afferent 

 fibres which enter the nerve-cord, the skin is diffusely 

 sensitive. In a few species the skin is slightly phosphor- 

 escent. The chitinous bristles, which are longest on the 

 genital segments, are much curved, and lie in small sacs 

 of the skin, in which they can be replaced after breakage. 



Muscular system and body cavity. — The earthworm 

 moves by the contraction of muscle cells, which are 

 arranged in circular hoops and longitudinal bands under- 

 neath the skin. The special muscles about the mouth 

 and pharynx have considerable powers of grasping, while 

 less obvious muscular elements occur in the wall of the 

 gut, in the partitions which run internally between the 

 segments, and on the outermost portions of the excretory 

 tubes. 



Unlike the leech, the earthworm has a very distinct body 

 cavity, through the middle of which the gut extends, and 

 across which run the partitions or septa incompletely 

 separating successive segments. In this cavity there is 

 some fluid with cellular elements, of which the most 

 numerous are yellow cells detached from the walls of the 

 gut. Possible communications with the exterior are by 

 the dorsal pores, and also by the excretory tubes, which 

 open internally into the cavities of the segments. 



Nervous system. — ^Along the middle ventral line lies a 

 chain of nerve-centres or ganglia, really double from first 



