THE ANATOMY. COELOM 31 



opening into the body-cavity from the exterior undoubtedly suggest nephridial organs ; 

 but no relations are apparent between the dorsal pores and the nephridia. 



The head pore is present in the Enchytraeidae, Naidomorpha and Lumbriculidae ; 

 its position differs: sometimes it is at the very tip of the prostomium, sometimes at 

 the junction of the prostomium with the buccal segment. A head pore has not been 

 described in any earthworm. As to the function of these pores Michaelsen thinks 

 that the head pore acts as a kind of safety valve to prevent undue pressure upon 

 the brain when the movements of the body force an unusual amount of coelomic 

 fluid into the anterior end of the worm's body. The dorsal pores he thinks have 

 the function of moistening the body and preventing its becoming unduly day ; it is 

 certain that the coelomic fluid is pressed out through the pores ; and their occlusion 

 is regulated by longitudinal muscles which pass from the margin of one pore to that 

 of the pore lying behind. Spencee even thinks that it is used for the purpose of 

 rendering the burrow of the worm sufficiently damp for it to move with comfort in; 

 possibly some coelomic fluid is forced into the cocoon by the movements of the body 

 when this is passed over the head. This latter function seems to be a likely one; 

 the use of the coelomic fluid as a lubricant seems to be, so to speak, too expensive, 

 especially when there are glands in the skin which appear to serve the same purpose. 

 It is not impossible that there may be an analogy between the dorsal pores and the 

 nephridia on the one hand, and the vertebrate kidney on the other. In the kidney 

 there seems to be a purely filtering action at the extremity of the renal tubules and 

 a secretory activity in the glandular section of the same tubes ; perhaps in the 

 Oligochaeta the dorsal pores pass out the waste fluids while the remaining excretory 

 products are elaborated and passed out by the nephridia. 



IV. Nephridia. 



The excretory organs of the Oligochaeta, to be treated of in the present section, wiU 

 be termed ' nephridia ' after the convenient name introduced by Lankester ^ ; the older 

 name of ' segmental ' organs — used by Williams, though it survives in many text 

 books, is not so useful, since it tends to disguise the real nature of the organs in 

 question ; moreover the term ' segmental ' suggests that they are always metameric in 

 arrangement, which is not invariably the case ; and that they are present in every 

 segment of the body, which is also as far from being the truth. 



§ I. Excretory organs in the embryo. Recent researches on the development of 

 the Oligochaeta, particularly those of Vejdovsky (9), have shown that at various 



' Notes on Embi-yology and Classification, Q. J. Micr. Sci. 1875. 



