16 MORPHOLOGICAL TYPES 



form of small bent tubes. Each nephridium bears bunches 

 of flame-cells or solenocytes, like hollow pins with a whip or 

 flagellum hanging down inside from the head, and serving to 

 flush out the contents. There is no internal opening to 

 the nephridia, which derive the products which they excrete 

 from the blood-vessels and ccelomic fluid by diffusion. The 

 nephridia open into the atrium (that is, morphologically to 

 the outside) by small pores situated near the top of the 

 secondary gill-bars. They are segmental in origin. 



There is another nephridium at the front of the animal, 

 lying dorsal to the oral hood near the middle line. It opens 

 into the pharynx just behind the mouth, and is known as 

 Hatschek's nephridium. No chordates other than Amphioxus 

 are known to possess nephridia. 



Genital System. — The sexes are separate, but very similar 

 in appearance. The gonads are pouches of germ-cells 

 arranged in a row on each side of the body from about the 

 ioth to the 36th segments, in the region of the gill-slits. 

 When these pouches are full they bulge into the atrium ; but 

 they must not be considered as lying in the atrium, for they 

 are separated from it by the whole thickness of the body- wall. 

 The segmental arrangement of the pouches is more or less 

 preserved. When ripe, the germ-cells burst out of the 

 pouches and pierce the body- wall, thus finding themselves in 

 the atrium. From here they make their way to the outside 

 through the atriopore. The cavity of the pouches is, of 

 course, ccelomic. 



Skeleton. — Reference has already been made to the skeletal 

 supports of the buccal cirrhi and to those of the gill-bars. 

 The most important skeletal structure of Amphioxus is, of 

 course, the notochord. This elastic rod extends from end to 

 end of the animal, dorsal to the gut and ventral to the nerve- 

 cord. Its extreme extension, almost to the tips of the anterior 

 and posterior fins, is noteworthy. 



Nervous System. — The central nervous system consists 

 of a straight tube running all the way down the back of the 

 animal, dorsal to the notochord and ventral to the fin-ray 

 boxes. Kolliker's pit on the left side of the snout represents 



