308 THE EARTHWORM AND OTHER ANNULATA 



the space between the body and the girdle. At the openings of the 

 seminal receptacles on segments nine and ten, spermatozoa derived 

 from another worm enter the space containing the ova, and fer- 

 tilization occurs. There is also included an albuminous secretion 

 from the skin glands which later serves as nutrient for the develop- 

 ing embryos. When the girdle is finally " laid," by being slipped 

 over the head, its two ends come together and it thus forms the 

 egg capsule, or cocoon, in which the fertihzed eggs develop (Fig. 

 143 C). The capsules of L. terrestris are deposited in the earth, 

 usually a few inches below the surface. They are oval bodies, 

 showing at either end the signs of their origin as a girdle-like 

 structure. They are about 6-8 by 4-6 mm. in size and of a 

 dark olive color. In L. terrestris, only one egg finally develops out 

 of several that are originally included in the capsule. In a related 

 species, L. communis, two worms arise from a single ovum, by 

 division of the embryo at an earty stage, in a manner comparable 

 with the production of "identical" twins in some of the mam- 

 malia. 



The earthworm, therefore, reproduces exclusively by the sexual 

 method. The structure and mode of action of the reproductive 

 organs have been described (c/. p. 295). The ova are formed 

 in ovaries. The spermatozoa are detached from the testes in an 

 immature stage and their maturation takes place in the seminal 

 vesicles, from which they eventually reach the outside by way 

 of the vasa deferentia. Sexual union consists in the mutual 

 exchange of spermatozoa, which are then stored in the seminal 

 receptacles. In this manner cross fertilization is insured at the 

 time of egg laying. The fertilized eggs develop to young worms 

 enclosed within an egg capsule, which contains an albuminous 

 fluid that serves as nourishment for the embryo. 



Development occurs as shown in Fig. 148. In addition to 

 ectoderm and endoderm, as in a diploblastic animal like Hydra 

 (cf. p. 260), mesoderm appears as in triploblastic animals. The 

 ccelome, which is differentiated in the mesoderm, is a new feature 

 that is not found in the Ccelenterata. The earthworm is thus 

 triplohlastic and ccelomoccelous (cf. p. 240). The relationships 

 between the germ layers of the earthworm and the cells of the adult 

 organism are shown by the accompanying table (Fig. 149). 



