vi CLASSIFICATION 349 



two worms afterwards separate. The clitellum then 

 secretes a tough chitinous tube or cocoon, 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 entirely withdrawn itself from the cocoon, the latter 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 segmentation. 



The cells of the polyplast soon become differentiated 

 into an outer ectoderm, and an inner endoderm enclosing 

 the enteron, 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 ecto- 

 derm (compare p. 209) ; 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- ii). 



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, etc.) are very differ- 

 ent from the earthworm 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, etc. In all these the body is elongated, 

 bilaterally symmetrical, and divided into metameres ; the 



