PHYLUM ANNELIDA 



211 



to the seminal receptacles of the mate, where they are stored. In 

 the meantime, the clitellum of each individual secretes a band 

 which binds them together at these two points. After each has re- 

 ceived sperms from the other, they separate by working themselves 

 through the bands secreted by the clitella. This leaves each animal 

 with a band which is gradually worked off toward the anterior end. 

 As the band passes over the openings of the oviducts eggs are re- 

 leased into it, and as it passes the openings of the seminal recep- 

 tacles sperms which came from the reproductive mate are released. 

 Both ends of the band close, forming a cocoon in which fertiliza- 

 tion and development take place. 



Fig. 111. — Regeneration and grafting in earthworm. A, anterior five segments 

 regenerated from posterior part of a worm ; B, tail regenerated from the posterior 

 portion of a worm ; C, tail regenerated from an anterior piece of a worm ; D, 

 union of three pieces to make a long worm ; E, grafting of two pieces to form a 

 double bodied worm ; F. anterior and posterior pieces grafted together to form a 

 new worm. The stippled areas in A, B, and C represent regenerated tissue. (From 

 Hegner, College Zooloc/y, published by The Macmillan Company, after Morgan.) 



Cleavage in earthworms is of the unequal holoblastic type. Soon 

 after the segmentation cavity is formed, a certain cell, known as the 

 mesohlast cell, is set off, and the cells resulting from its divisions 

 move into the cleavage cavity, and will form the mesoderm. As the 

 mesoblast cells move into the cleavage cavity, gastrulation occurs 

 by invagination to form the endoderm and ectoderm. The gastrula 

 elongates and the archenteron opens at both ends to form the mouth 

 and anus. The mesodermal cells which fill the space, between the 

 ectoderm and endoderm develop segmental cavities which are the 

 coeloms of the metameres. At this stage the animal constitutes a 

 tube within a tube and from it the organs of the adult develop. 



