THE EARTHWORM 117 



cellular structure of each of the layers. Label the layers, indicate what 

 tissues are found, and the functions of all parts. 



(g) Study the nerve cord in the cross section, and note the outer 

 layer, containing muscle cells and blood vessels; the three large, clear 

 areas, or giant fibers; the nerve fibers and the nerve cell bodies. 

 Understand from lectures, textbook, and Fig. 57 the relationship be- 

 tween the central and peripheral portions of the nervous system and 

 the cellular organization of the system as a whole. 



Exercise 10. — Comparison of the Cell-layers and Tissues in the 

 Earthworm, Hydra, and Frog. 



(h) Compare the cellular structure, layer for layer, in correspond- 

 ing parts of the earthworm, hydra, and frog. This may be done in 

 tabular form, indicating the layers and tissues and their origin from 

 the three germ layers in development. Write a short statement pointing 

 out the similarities and differences in the body-plans and in the de- 

 gree of somatic cell specialization found in these three animals. 



IV. REGENERATION, REPRODUCTION, AND DEVELOPMENT 

 Exercise 11. — Regeneration. 



(a) When earthworms are collected in nature individuals are often 

 found with posterior ends showing a varying number of regenerating 

 somites. Experimental study of this regeneration shows that many 

 somites can be restored posteriorly and that in general the number 

 thus regenerated is in proportion to the number removed. At the an- 

 terior end, however, the power of regeneration is restricted. Typically, 

 only four somites can be regenerated and even these will not be nor- 

 mally restored if as many as fifteen somites have been removed. Some 

 species of annelids have much greater powers of regeneration. 



(b) The small fresh-water annelid, Tubifex tubifex, somewhat re- 

 sembles the earthworm in its powers of regeneration. Examine, in 

 water in a watch glass, recently collected Tubifex that show posterior 

 regeneration ; anterior regeneration is seen occasionally. If the regen- 

 erating individuals are abundant it should be possible to arrange a 

 series that will illustrate stages of posterior regeneration. The stages 

 of anterior regeneration cannot be easily observed in such living ma- 

 terial. To study the posterior regeneration experimentally, remove the 

 posterior halves of several individuals by a transverse cut made with 

 a sharp scalpel; set aside the pieces, as directed by instructor, for 

 subsequent examination. In the course pf two weeks at room tempera- 

 ture the more important changes will occur. During such regeneration 

 the cut end heals and forms a new anal opening within forty-eight 



