322 INVERTEBRATE MORPHOLOGY. 



■while tactile setae or papill* are scattered over the bodj. In 

 those forms which possess a lateral nerve sense-organs re- 

 sembling those of Capitella occur metamerically along it, and 

 in the genus Slavina are increased in number so as to form a 

 circle of from fifteen to twenty papillae surrounding each 

 metamere and innervated by a branch from the lateral nerve. 

 Cup-shaped organs, supposed to be gustatory, occur especially 

 abundantly on the prostomial metamere. 



The excretory system has usually a typically metameric 

 arrangement — a single pair of coiled tubules lying in each 

 metamere. Each tube opens by a ciliated funnel into one 

 coelomic compartment and then passes backwards, perforating 

 the dissepiment, into the next succeeding compartment in 

 which the coiled portion lies, and opens to the exterior in 

 this metamere between the dorsal and ventral rows of setae. 

 The lumen of the coiled portion of the tubule is intracellular, 

 the tubule consisting in this region of a series of perforated 

 cells recalling the condition found in the Platyhelminths. In 

 a certain number of anterior metameres the nephridia may 

 be wanting in the adult condition, though in younger stages 

 provisional nephridia are to be found in these metameres 

 later disappearing. 



Considerable variation is to be found in the nephridial system of the 

 Oligocheeta, some of the variations suggesting important theoretical con- 

 siderations. A head-kidney similar to that described as occurring in the 

 Troohophore larva persists in the adult stage in some Oligochseta, and in 

 Ctenodrilus appears to be the only nephridium which exists. In other 

 forms, such as Chaetogaster, the entire nephridial system is composed of 

 tubules having a decided similarity to the head-kidney in the intracellular 

 character of the lumen, and in the absence of any ciliated funnel, the 

 inner end of the tubule being closed. Furthermore the cells enclosing the 

 canal are in addition perforated by numerous minute branching canals 

 which open into the central lumen. This fact suggests the complete 

 homology of the nephridial system throughout the entire body notwith- 

 standing the usual marked histological distinction between the head- 

 kidneys and the nephridia of the trunk metameres. There seems little 

 reason to doubt that the Oligochasta have been derived from the Poly- 

 chaeta, and the nephridial system in the two forms is, therefore, homol- 

 ogous. It must, therefore, be possible for a nephridium with an intra- 

 cellular canal to be transformed into one in which the canal is intercellular. 

 The nephridia of Ohcetogaster are unquestionably homologous with those 



