XVIIl] 



EXCRETORY ORGANS 



401 



m 



- P 



-e 



the peripharyngeal bands and the inner sides of the gill-bars. 

 Here it is caught by the inrushing current of water produced by 

 the cilia of the oral cirri and swept 

 back along the hyperpharyngeal groove 

 to the opening of the intestine, entang- 

 ling in its passage the small plants and 

 animals carried by the water; the latter 

 of course escapes into the atrial cavity 

 through the hundred or so long narrow 

 gill-slits. The intestine is prolonged for- 

 ward on the right side of the pharynx 

 into a blind pouch, the so-called liver 

 (/, Fig. 197), which probably secretes a 

 digestive juice. 



The excretory organs of Amphioxus 

 are small and have only recently been 

 discovered. We have seen that in the 

 region of the pharynx the coelom has be- 

 come reduced to a narrow canal, beneath 

 the pharynx, and to two dorsal canals at 

 the sides of the notochord (Figs. 189, 190). 

 At the level of each tongue-bar there 

 projects into each of these canals a true 

 nephridium of the type described in 

 Platyhelminthes and Annelida. Each 

 nephridium opens into the atrial cavity 

 below by a short wide tube. Internally it 

 divides into several branches which end 

 blindly, each branch terminating in a tuft 

 of those peculiar stalked, flame cells 

 known as solenocytes (Figs. 195, 198). 

 One peculiarly large nephridium known as 

 "Hatschek nephridium" lies in front of 

 the velum above the buccal cavity but 

 below the notochord. This nephridium 

 opens into the pharynx. Besides these a 

 number of thickened patches of the atrial 

 epithelium, discovered by Johannes Muller 

 and called by him renal papillae, are 

 thought to assist in excretion (Figs. 188 

 and 190). 



s. & M. 26 



-an 



FIG. 197. Amphioxus dis- 

 - sected from the ventral 



side x 2. After Rathke, 



slightly altered. 



an. Anus. at. Position of 

 atrial pore; the extension 

 of the atrium behind this 

 point is indicated by the 

 dotted line passing over 

 to the right side of i, the 

 intestine, e. Endostyle. 

 g. Gonadic pouches. I. 

 Liver, m. Entrance to 

 mouth with the oral cirri 

 lying over it. p. Phurynx. 



