THE CCELOMATA INVERTEBRATA 171 



invaluable to the plant life. Its burrows allow of the percolation 

 of the rain to the deeper layers, and also of the air. Darwin, in his 

 masterly book on " The Formation of Vegetable Mould through the 

 Action of Earthworms/' estimates that by their castings they bring 

 to the surface more than ten tons of the deeper soil per acre per year, 

 and so gradually cover up stones, etc., that may be lying upon it. 

 They also pull in to line their burrows leaves, which, of course, 

 disintegrate, forming a rich mould. 



The earthworm's body is long and cylindrical, running off into 

 a bluntly pointed anterior end and a somewhat flattened obtusely 

 truncated posterior end, and its greatest diameter is about one- 

 third of the way from the anterior end. It reaches a maximum 

 length of about seven inches. The general colour is a pinkish-brown, 

 but it is much darker above than below. As it always travels with 

 the same surface to the ground, we can distinguish a ventral and a 

 dorsal surface, the ventral being slightly flatter than the dorsal. 

 Thus it exhibits a definite orientation not only with front and hinder 

 ends, dorsal and ventral surfaces, but also with right and left sides, 

 which are similar to one another, so that it is a bilaterally symmetrical 

 animal. 



A series of transverse ring-like grooves divides the body off into 

 about 150 segments, somites or metameres, which are larger in front 

 than at the hinder end. We also find on dissection that this 

 segmentation is not superficial, but that the inside of the animal 

 exhibits clearly the repetition of the essential organs of the body in 

 compartments of the crelom, separated off from one another by 

 transverse partitions or septa, which coincide with the grooves on 

 the outside. In its general characters Lumbricus agrees with a large 

 number of other worm-like forms, which from their ringed bodies 

 are classed in the Phylum Annelida or Annulata. This repetition 

 of a number of parts in a series, so very strongly marked in the 

 Annelids, is a morphological feature of considerable importance, and 

 an animal so built up is said to be metamerically segmented, while 

 the repeated parts are described as serially homologous. It is a 

 feature marked even in the higher animals like the dogfish, and 

 distinct traces of it are still to be found in the frog, rabbit, and 

 ourselves. 



At the extreme anterior end of the earthworm is a blunt lobe, 

 the prostomium, not homologous with the somites. Behind it, the 

 next part if the worm, the first true somite, surrounds the ventrally 

 situated mouth, and so is termed the peristomium. On the dorsal 

 surface a backward prolongation of the prostomium is dovetailed 

 into the peristomium ; in Allolobophora the projection extends 

 completely across the peristomium, but in Lumbricus it only goes 



