GENERAL ZOOLOGY 



9 11 15 



32 37 



MMMm 



32 26 A 15 11 9 



^ b 



Fig. 14.12. Copulation and 

 cocoon formation in the 

 earthworm. A, two worms 

 encased in a sheath of mucus 

 secreted by epidermal glands, 

 as spermatozoa are exchanged 

 along the seminal grooves 

 (arrows). B, transverse sec- 

 tion, showing mucus sheath 

 and four seminal grooves (g), 

 two on each worm. C, forma- 

 tion of the cocoon or egg 

 capsule by the clitellar glands 

 of one individual; the cocoon 

 passes anteriorly (a to b) until 

 it is slipped off as the com- 

 pleted capsule, containing 

 several zygotes. Certain of 

 the somites are numbered for 

 orientation. 



the girdle is slipped off over the anterior end of the worm; its ends close, 

 and it becomes an egg capsule, containing a nutrient fluid and several zygotes 

 which develop into miniature earthworms before emerging. The course of 

 embryonic development (Fig. 14.13) is much modified as compared with that 

 oi Nereis and other polychaetes; there is, of course, no stage corresponding to 

 the free-swimming trochophore of the marine forms. In its main features, 

 such as type of cleavage, mode of origin of mesoderm and coelom, and 

 development of segmentation, however, the embryogeny of the highly special- 

 ized earthworm falls in line with that of the more generalized annelids. 



Regeneration. Earthworms have a considerable ability to replace whole 

 somites or entire groups of somites by a process of regeneration of lost parts. 

 It is not surprising, considering the greater specialization of the anterior end, 

 and the fact that in ordinary growth somites are regularly added at the 

 posterior end, to find the capacity of regeneration more restricted anteriorly 

 than posteriorly. For example, when pieces are cut from the posterior end of 

 the "manure worm," AUolobophora foetida, the animal forms a few new somites 

 and then others, by addition, just as somites are normally added posteriorly 

 during growth. When somites are removed at the anterior end, however, a 

 maximum of only five can be regenerated. A new head may be formed, but 

 the individual never restores the full number if more than five have been 

 removed. When the region containing the reproductive organs is removed, 

 it is not regenerated. In Tuhijex luhife.x, a common, small, fresh-water oli- 

 gochaete, regeneration is extensive and rapid; an average of 31 new somites 

 may be formed in 32 days. This regeneration has been shown to occur largely 

 through the activities of totipotent cells, or neoblasts, which migrate to the 

 region of injury and differentiate during regeneration (see the discussion of 



412 



