II. Q 



EXCRETION OF AMPHIOXUS 



35 



these, through their cilia, do much of the work of the body, the 

 blood actually leaves the gills less rich in oxygen that when it enters 

 them. Oxygenation probably takes place chiefly in the lacunae close 

 to the skin, perhaps especially those of the metapleural folds. 



9. Excretory system of amphioxus 



One of the most mysterious features about the organization of 

 amphioxus is that there are flame-cells, comparable with those found 



Fig. 12. Solenocytes of amphioxus, showing the nuclei, long flagella, and the openings 

 into the main excretory canal, which leads to the atrium. (After Goodrich.) 



in platyhelmia, molluscs, and annelids. The excretory organs, there- 

 fore, do not conform to the basic chordate plan, and are in fact very 

 different from those not only of all other chordates but also from any 

 found in the remote invertebrate allies of the chordates which, as we 

 shall presently see, include the echinoderms, brachiopods, and polyzoa. 

 The nephridia lie above the pharynx. To each primary gill bar 

 there corresponds a sac, opening by a pore to the atrium and studded 

 with numerous elongated flame-cells (solenocytes) (Fig. 12). These 



