EARTHWORMS 169 



animals. The acids present in the top soil find a ready path down 

 their burrows, and so gain access to the solid rock beneath, and 

 eat it away. While the " casts/' whether thrown up on level 

 or sloping surfaces, are dispersed by the wind or washed to lower 

 situations by the rain, and thus the general attitude of con- 

 tinents is slowly but surely reduced. 



There are in Britain twenty species of earthworms belonging 

 to the family Lumbricidce, and contained in five different genera. 

 The features by which these genera are distinguished from one 

 another are in many cases internal and not visible except after 

 dissection. The external characteristics concern the shape of 

 the upper lip (prostomium), the position of the clitellum, and of 

 certain swellings termed " tuber cula pubertatis " upon it, and 

 the paired or unpaired arrangement of the bristles (setae), the 

 site of the male apertures, and a few other minute features whose 

 determination is too difficult for treatment here. 



The " brandling " (Eisenia foetida} is of frequent occurrence in 

 dung heapG and richly manured soils. Its general colour is red, 

 marked with purple or brownish-yellow rings. The length is 

 about 3^ inches. The prostomium extends into but not across 

 the first segment. The clitellum extends usually from the 26th 

 to 32nd segments, the tubercula pubertatis from the 28th to 

 3oth or 3ist. The setae are closely paired. The male apertures 

 in the i5th segment. Three other common earthworms for 

 which there are no popular names are 



Allolobophora longus, A. chloroticus, and Lumbricus terrestris. 

 The genus Allolobophora is distinguished externally from Lum- 

 bricus by the fact that in the former the 

 prostomium does not, while in the latter 

 it does, extend back so as to touch the 

 furrow which divides the ist from the 2nd 

 segment. Both alike have the male aper- A 



tures in the I5th segment. FIG. 62. Prostomium and ist 



A. longus is smoky pink-grey in colour ; segments of- A, Aiido- 



, . i bophora ; L, Lumbricus. 



from 5 to 7 inches long, and is common in 



cultivated soil. Its clitellum extends from the 27th and 28th 

 segments to the 35th ; the tubercula pubertatis from the 32nd 

 to 34th. The setae are closely paired. 



