INTERNAL ANATOMY. 



79 



certain resemblances to that of Amphioxus, in that it 

 occurs in the form of distinct segmental tubules, or 

 nephridia, each possessing a funnel-shaped opening into 

 the body-cavity, and an opening to the exterior at the sur- 

 face of the body. 



It was, in fact, the recognition, some twenty years ago, 

 by Semper and Balfour, of the resemblance between the 

 arrangement of the nephri- 

 dia of the Annelids and 

 the primary segmental ori- 

 gin of the kidney of the 

 Craniota that was chiefly 

 instrumental in placing the 

 Annelid-theory of Verte- 

 brate descent on a tempo- 

 rarily firm basis. 



A dissection of the an- 

 terior portion of the body 



of an earthworm, exposing ^.^ g^,. Anterior portion of earth- 



the nephridial tubules, is wo™ dissected open from above to show 



the nephridia and nervous system. (From 

 shown in Fig. 39. A pair w. T. Sedgwick and E. B. WILSON'S 



of such convoluted tubules G.««a/ sw.-i'j..) 



pr, Prostomium (prseoral lobe), e.g. 

 occurs in each segment, or Cerebral ganglion, which has receded from 



the prostomium from the ectoderm of 



ring, of the body, com- which it arose, com. Circumcesophageal 



mencing from the third, commissure surrounding the buecal tube 



t> (latter not represented). v.n.c. Ventral 



Physiologically, of course, nerve-cord. n. Segmental nerves. nph. 



Nephridia. sp. Dissepiments. 



they are directly com- 

 parable to the renal tubules of the Chordata, and in their 

 general features, allowing for the absence of a common 

 duct, the similarity in the two cases is striking enough. 

 But when this undoubted similarity is used as an argument 

 for deriving the Vertebrate excretory system directly from 

 that of the Annelids, we tread on very uncertain ground. 



