20 COMPARATIVE ANATOMY 



those of the rest of the body, being finer and nearly straight, 

 with hooked inner ends. There is also a pair of modified 

 chseta in somite 15. 



In addition to the mouth and anus, the following external 

 apertures musfbe noted: — (i) The dorsal pores, a series of 

 small round apertures situated in the mid-dorsal line in the 

 grooves separating the somites from one another. In 

 L. heradeus the first dorsal pore occurs in the groove between 

 the eighth and ninth somites, and there is one in every sub- 

 sequent intersegmental groove. They open directly into the 

 body-cavity, and their exact function is not known. (2) The 

 oviducal pores, or female apertures, are minute round pores 

 placed one on each side of somite 14. (3) The spermiducal 

 pores, or male apertures, are relatively large slit-like openings 

 with tumid lips placed one on each side of somite 15. 

 (4) The spermathecal pores are two pairs of minute round 

 openings placed in the intersegmental grooves separating 

 somites 9 — 10 and 10 — 11 at the level of the uppermost 

 member of the lateral couple of ch^tse. (5) The nephridio- 

 pores are the minute openings of the excretory tubes or 

 nephridia. There is a pair on every somite except the first 

 three and the last, each opening being situated just in front 

 cf the uppermost member of the ventro-lateral couple of 

 chaetfe. The nephridiopores are hard to see, but they can 

 usually be made apparent by gently squeezing the body of 

 the worm, when small droplets of liquid exude from them. 



The earthworm has no obvious organs of special sense, and 

 no external appendages except the chsetse. Darwin's experi- 

 ments show that earthworms are sensitive to light, but not 

 to sound, though they are very sensitive to any jar or vibration, 

 retreating at once into the depth of their burrows when the 

 ground near them is shaken. They also appear to have some 

 sense of smell. It will be shown later that the epidermis 

 is provided with a great number of minute sensory organs, 

 far too small to be recognised by the naked eye. 



On opening the worm, which is best done by making a 

 longitudinal incision in the mid-dorsal line and pinning out 

 the walls of the body right and left, many of the more 

 important features of its internal anatomy can be seen without 

 further dissection. The gut is a straight tube running from 

 the mouth to the anus, and separated from the body-wall 



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