340 THE EARTHWORM chap 



which forms a broad ring round the body in this region, and 

 which is gradually slipped forwards. As it passes over the 

 apertures of the oviducts and spermothecee, ova and sperms 

 (the latter derived from the other individual) are passed into 

 it, as well as albumen secreted by certain glands present in 

 this region. When the worm has entirely withdrawn itself 

 from the cocoon, the latter closes up at the ends in virtue of 

 its elasticity, and the eggs, after fertilization, undergo seg- 

 mentation. 



The cells of the polyplast soon become differentiated into 

 an outer ectoderm and an inner endoderm enclosing the 

 archenteron, which communicates with the exterior by the 

 blastopore (compare p. 201). A mesoderm (p. 202) is then 

 developed, and each layer gradually gives rise to the cor- 

 responding parts in the adult animal, much as in the frog 

 (compare p. 209), except that the greater part of each 

 nephridium is apparently derived from the ectoderm, only 

 the inner end of the tube, not the whole of it, arising from 

 the mesoderm : the mesoderm undergoes segmentation, 

 the coelome appearing in it as a cavity (p. 203) — or rather 

 as a series of cavities, one in each segment. The young 

 worm is then hatched, and it is to be noticed that it passes 

 through no metamorphosis (p. 11). 



In the marine worms belonging to the same class as the earthworm, 

 on the other hand, the young is hatched in the form of a larva known 

 as the trochosphere, which swims by means of cilia arranged in circles 

 round the body, and gradually undergoes metamorphosis into the adult 

 form. 



There are a number of different kinds of animals 

 commonly known as "worms," but many of these (e.g. the 

 parasitic worms in the lungs and bladder of the frog, 

 the tapeworms, &c.), are very different from the earth- 

 worm in structure, and are placed in several different 



