iiO ANATOMY OF AMPHIOXUS. 



If we were to consider the excretory system apart from 

 the rest of the organisation, this would be the only course 

 to follow. But when the whole organisation is taken into 

 account, the only justifiable conclusion seems to be, not 

 that the Vertebrate renal system is to be derived from that 

 of the Annelids, but that, as Riickert suggests, both may 

 possibly have been evolved from a common starting-point. 



It is eminently probable that, in respect to this and the 

 other systems of organs, as well as the segmentation of 

 the body, the Annelids and Vertebrates present an in- 

 stance of parallel evolution. This will become more evi- 

 dent as we proceed. Those who uphold the so-called 

 Annelid-theory have no cause to complain of the absence 

 of a common duct to the nephridia, since this has been 

 found in some cases to occur. 



In 1884 Eduard Meyer discovered that in certain 

 marine Annelids {Lanicc conchilega and Loimia meeliisa) 

 belonging to the family of the Terebellid^, the nephridia 

 of each side were joined together by longitudinal ducts, 

 which he compared, though with great reserve, to the 

 mesonephric ducts of the Vertebrata.* In these worms the 

 nephridia do not occur in all the segments of the body, but 

 are confined to the anterior so-called thoracic region, their 

 number being very limited. In the thorax, the dissepi- 

 ments v/hich typically divide the segments from one 

 another are absent, so that the body-cavity would here 

 form a continuous uninterrupted space, were it not that it 

 is divided into two chambers, an anterior and a posterior, 

 of which the latter is the larger, by a muscular diaphragm. 

 In the anterior thoracic chamber (Fig. 40) there are three 

 pairs of nephridia which are united together on each side 

 by a short duct opening to the exterior by a single aperture. 



* This flisc(jvcr\' wcis alsn made later liut independently l>v T- T. Ci'.xmxg- 

 IIAM f(jr J.tniict: coiichilcii^a. 



