COMMON EARTHWORM. 197 



called or prostomium (infra) lies entirely in front of the mouth and corre- 

 sponds to the praeoral somite in o'ther Chaetopoda. The mouth is ventrally 

 placed in the second segment and is therefore subterminal. The anus on the 

 other hand is posterior and terminal. The whole body is distinctly divided 

 into a series of somites or segments, separated by well-marked intersegmental 

 furrows. Vertical transverse septa or dissepiments which divide the body 

 into compartments, correspond internally to these furrows with the excep- 

 tion of the first five or six. The compartments communicate with one 

 another round the supra-nervian blood-vessel. The chitinoid cuticle has 

 been loosened by maceration in the spirit and may be seen lying in folds 

 in the posterior region of the body. In life it is iridescent owing to the 

 presence of two sets of fine parallel superficial lines crossing each other at 

 angles of 75 to 80. On the ventral surface of the fifteenth somite count- 

 ing the praeoral as the first, are two white tumid elevations. These cor- 

 respond to the apertures of the vasa deferentia. If the worm be held so 

 that the light falls upon it obliquely, two somewhat raised lines may be 

 seen running down the body on each side parallel to one another. The 

 more ventral of these lines corresponds very nearly in position with the 

 apertures of the vasa deferentia. Both lines indicate the position of the 

 setae which constitute the locomotor apparatus of the Chaetopoda. There 

 are in each somite of the common Earthworm two setae in each line, 

 implanted a small distance apart, but they are often lost by accidents. The 

 outer row is wanting in some instances as far back as the clitellum (infra). 

 The inner row usually commences on the fourth or fifth somite. Both 

 rows may be absent in the posterior somites. The spot corresponding to 

 the inner or ventral row on the twenty-sixth somite of this specimen is 

 somewhat swollen. The setae are here, as in the region of the clitellum, 

 peculiarly long and delicate in a sexually mature worm, and are generally 

 retracted. They are supposed to act as accessory copulatory organs. 



The dorsal and lateral parts of somites thirty-one to thirty-eight, 

 and especially of somites thirty-three to thirty-six, are white and swollen, 

 and the swollen region is bordered by a prominent well-marked edge on 

 either side of the median ventral line. These swollen somites constitute 

 the clitellum, an organ especially characteristic by its great development 

 of the terrestrial Oligochaeta. It is glandular and secretes a plentiful 

 mucus from which the cocoon is formed. The prominent ventral edge 

 acts as a copulatory organ. The development of the clitellum depends on 

 the age and the state of sexual activity of the individual. 



Vejdovsky believes, from his observations on the growth of the prostomium or 

 praeoral somite in the individual produced by fission of Aeolosoma tenebrarum as 

 well as in embryoes of Rynchelmis, that it is really an outgrowth of the buccal 

 somite and not a separate somite. In Aeolosoma the buccal somite developes the 



