x PHYLUM ANNULATA 481 



i i a la and allies) there is a single pair of large nephridia in the 

 anterior region of the body, with smaller pairs in the posterior 

 segments, the former alone appearing to have an excretory function 

 while the latter act exclusively as genital ducts. In Sternaspis 

 only a single pair of nephridia are present, which, though they have 

 small ciliated funnels, are not known to communicate with the 

 exterior. 



In the Oligochaeta the nephridia are usually simple, elongated 

 and coiled tubes, a pair or sometimes more than one pair in each 

 segment ; but in some, these are replaced or supplemented in 

 certain of the segments, or in all, by a branching system of tubes 

 with or without ciliated funnels. Sometimes the ordinary nephridia 

 are not developed in the segments lodging the reproductive organs, 

 their place being there taken by three pairs of tubes of the nature 

 of localised coelomoducts which become modified to give rise to the 

 reproductive ducts ; but ordinary nephridia may be present in these 

 segments as well. In some Oligochseta the nephridia of the most 

 anterior segments open into the mouth or pharynx, and have 

 apparently taken on the function of digestive glands (peptoneph- 

 ridia), and all the nephridia of the posterior region of the body 

 in one species (Allolobophora antipce), instead of opening on the 

 exterior, communicate with a pair of longitudinal canals which 

 posteriorly open into a median 'vesicle communicating with the 

 rectum. 



The permanent nephridia of the adult Chaetopod are preceded 

 in the larva by a series of provisional or embryonic nephridia of a 

 temporary character. These have been found to occur in the head 

 (prostomiuin) of many larval Oligochseta and Polychaeta. They are 

 ciliated intracellular tubes, sometimes branched, which do 

 not open into the cavity of the prostomium. Sometimes soleno- 

 cytes occur at the inner ends of the branches or of the undivided 

 tube. Embryonic nephridia have also been shown to occur in the 

 body in certain forms. 



Phosphorescence, the production of light rendering the 

 animal brilliantly luminous in the dark, occurs in a few cases 

 (various Polynoids, Chcctopterus, &c.). 



In the arrangement of the reproductive organs in the 

 Chtetopoda there is an essential difference between the two sub- 

 classes, the Oligocha?ta being hermaphrodite, and the Polychseta, 

 with only a very few exceptions, unisexual. In the latter the 

 gonads, ovaries or testes as the case may be, are masses of cells 

 which are developed as the result of a proliferation of the 

 coelomic epithelium in certain positions (Fig. 378). Usually these 

 organs, which are only conspicuous about the breeding season, occur 

 in the great majority of the segments of the body ; sometimes they 

 are confined to a certain region. The exact place which they 

 occupy in the interior of the segment varies in different cases : 



VOL. i i I 



