94 MANUAL OF ANIMAL BIOLOGY 



anterior end of each worm. This hardens somewhat and with the 

 aid of additional material secreted by certain specialized ventral 

 glands holds the two animals securely together. 



Sperm is now discharged by each worm from the openings of 

 the sperm ducts on segment xv. It is forced posteriorly, under the 

 slime tube and in a fine groove in the body wall, to the clitellar 

 region. Here the sperm is passed into the openings of the seminal 

 receptacles of the other worm which, it will be remembered, are 

 being held in close apposition. After the exchange of sperm the 

 worms break loose from their coverings and separate. 



In each animal certain glandular cells of the clitellum now 

 become active and secrete mucus material to form a resistant, 

 hollow, elastic ring entirely around the body, which is filled with a 

 jelly-like nutrient substance also secreted by clitellar cells. This 

 band is then gradually worked forward by contractions of the body 

 wall. It receives a few egg cells as they are passed from the 

 oviduct openings at xiv, and sperm, previously received from 

 another individual, at the openings of the seminal receptacles at 

 ix/x/xi. The band, now containing eggs and sperm, is moved 

 further anteriorly, and, finally, after passing entirely from the 

 animal, contracts to form a small rounded egg case, in which the 

 eggs, fertilized by the foreign sperm, develop. 



After fertilization the eggs begin to divide. The cleavage of the 

 egg of an Earthworm is somewhat unequal and irregular, but there 

 are soon formed, just as in the development of Hydra, a hollow 

 sphere, the blastula, and then a two-layered gastrula, the wall of 

 which is composed of an outer ectoderm layer and an inner endo- 

 derm layer. Soon mesoderm is formed between the ectoderm and 

 endoderm, and it is within the mesoderm, as previously described 

 (p. 66), that the coelom arises, which is so important a structural 

 feature of the Earthworm and the higher forms of animals. (W. 

 f. 173.) 



Asexual Reproduction by Regeneration. In the Earth- 

 worm, the normal method of reproduction is sexual, as has just 

 been described. This animal, however, shows great powers of 

 regeneration which under certain conditions may amount to asex- 

 ual reproduction. For example, if an Earthworm is cut trans- 

 versely through the middle of the body, each half will regenerate 

 almost completely. Other species of Annelida are known in which 

 the power of regeneration is even greater than in the Earthworm. 



