PHYLUM ANNULATA 



pelagic larva, the Trochosphere (Fig. 114), provided with one 

 or several circlets of cilia. 



When a common earthworm is compared with Nereis, 

 certain resemblances are at once discernible. The earth- 

 worm (Fig. 115) has the same elongated cylindrical body, 

 divided by ring-like grooves into a large number of seg- 

 ments or metameres. But the well-developed head-region 

 is absent, as are the eyes, palpi, and tentacles, and the 

 parapoda are not present, nor the dorsal and ventral cirri. 

 Setae, however, are present (Fig. 116), though so short as to 

 be distinguishable with difficulty ; two 

 double rows run along each side of the 

 ventral surface, so that there are alto- 

 gether eight of these short setae on each 

 segment. A thickened zone — the saddle 

 or clitellum — is to be observed extend- 

 ing over five segments, in front of the 

 middle of the body. In internal struc- 

 ture there is a considerable resemblance ; 

 but the reproductive organs are her- 

 maphroditic in arrangement and more 

 complex in structure than in Nereis. 

 There are two special male ducts or vasa Fig. 116. — Lumbricus, 



setse, highly magnified. 



deferentia, opening on the ventral sur- 

 face of the fifteenth segment, and female ducts or oviducts 

 opening on the fourteenth. 



The fertilised ova of the earthworm are enclosed, together 

 with a quantity of an albuminous fluid in a cocoon, the wall 

 of which is formed of a viscid secretion from the glands of 

 the clitellum, hardened and toughened by exposure to the 

 air. The cocoon is deposited in the earth, and the embryos 

 develop into complete, though minute, worms before they 

 make their escape. At a certain stage the embryos are 



