CHAPTER XXVII 



THE EXCRETORY AND REPRODUCTIVE SYSTEMS 



Amphioxus is unique among chordate animals in possess- 

 ing true nephridia. These organs are situated above the 

 gill-slits, their solenocytes project into the lateral dorsal 

 coelomic cavities, and their external openings lead into the 

 atrial cavity. The gonads of Amphioxus are segmental, and 

 situated at the ventral ends of the original myoccelic cavities. 

 The germ-cells of each segment make their way independently 

 to the exterior (actually into the atrial cavity) by pores in the 

 body- wall. 



It is possible that the region of the ccelom, which in all 

 higher Chordates is concerned with the formation of the ex- 

 cretory organs, corresponds to that region which in Amphioxus 

 forms the gonads. 



Originally there must have been a continuous row of little 

 tubes on each side of the body, leading out of the splanchnocoel 

 into a duct which collected from them all, and opened to the 

 outside at or near the anus. These little tubes represent the 

 original connexion between the myoccel and the splanchnocoel 

 (the nephrocoel, in the intermediate cell-mass), and consequently 

 they are segmental in arrangement : one tubule on each side 

 to each segment. Such an arrangement has been called an 

 archinephros, and the duct the archinephric duct, and this 

 condition is almost fulfilled in the Cyclostome Bdellostoma. 

 Here a continuous row of tubules is formed, but an inter- 

 mediate section of them disappears, thus separating an anterior 

 batch — the pronephros — from a more posterior set — the 

 mesonephros. In other forms the pronephros appears first, 

 and the duct which is formed by the backward growth of the 

 ends of the tubules is the pronephric duct. The pronephros 



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