200 



PH ] 7 L UM ANNELIDA, 



assist in locomotion. These bristles are fixed like pins into the ground, 

 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- 

 -,R.S niens is usually necessary. Almost 

 always plain on the 1 5th ring are 

 the two swollen lips of the male 

 ducts, less distinct on the I4th are 

 the apertures of the oviducts through 

 which the eggs pass, while on each 

 side, between segments 9 and 10, 

 10 and II, are the openings of two 

 receptacula seminis or spermathecce 

 into which male elements from 



wi .1- f .1 111 \.\J VVllJ.^rll llidlV.' V-lV-lllV-llLo 11 \J 111 



FIG. o5. Anterior region of earth- ., ,, -, r 



worm.-After Hering. another earthworm pass, and from 



which they again pass out to fertilise 



Note the eight setae (s.) on each segment, the eggs of the earthworm when 

 R.S., Spots between 9-10, 10-11, t T, p<; arp \~\A "Pirh spcrmpr^ rnn 

 indicate openings of receptacula tn . GSe aie la ^' f^ acft segment con- 

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

 on segment 14; i'd., openings of vasa which have minute ventral-lateral 

 deferentia on segment 15. 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 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 circular hoops and longitudinal bands 

 underneath the skin. The special muscles above the 



