1 84 



SEGMENTED WORMS OR ANNELIDA. 



at times so firmly that even a bird finds it difficult to pull the worm 



from its hole. As each of the four 

 longitudinal rows is double, there 

 are obviously eight bristles to each 

 ring. On the skin of the ventral 

 surface there are not a few special 

 apertures, which should be looked 

 for on a full - grown worm ; but 

 careful examination of several speci- 

 mens is usually necessary. Almost 



Fig. 8i. — Anterior region of earth- 

 worm. — After Hering. 



always plain on the 15 th 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 spermathecEe 



into which male elements from 



another earthworm pass, and from 



which they again pass out to fertilise 



Notetheeightsetae(.s.)oneachsegment. the eggs of the earthworm when 



^ifSca^Vntn^of^cepta^a these are laid. Each segment con- 



seminis ; Ovd., openings of oviducts tains a pair of excretory, tubes, 



on segment 14 : vd., openings of vasa which have minute ventral -lateral 



deferentia on segment 15. apertures, while on the middle line 



of the back, between every two 

 adjacent rings, there are minute pores, through which fluid from the 

 body cavity may exude. 



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 phosphorescent. The 

 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 hoops underneath the skin, and in longitudinal 

 bands more internally. The special muscles above the 

 mouth and pharynx have considerable powers of grasping, 



