vi CLASSIFICATION 349 



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 spermothecae, 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 When the worm has entirely withdrawn itself 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 fertilisation, 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 (pp. 202 and 

 322) is then developed, and each layer gradually gives rise 

 to the corresponding parts in the adult animal, much as in 

 the frog, except that the nephridia are apparently derived 

 in the first instance from the ectoderm (compare p. 209) ; 

 the mesoderm undergoes segmentation, the ccelome appear- 

 ing 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 meta- 

 morphosis (p. n). 



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, liver, 

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

 worm in structure, and are placed in several different 

 phyla. The earthworm is a member of the phylum 

 Annulata, which also includes a number of other worms living 

 in the sea and in fresh-water, as well as the leeches, &c. In 

 all these the body is elongated, bilaterally symmetrical, and 

 divided into metameres ; the integument is soft ; the mouth 



