THE CEPHALOCORDA 187 



nephridia on each side of the pharynx of Amphioxus. The 

 nephridia lie in the dorsal ccelom in close proximity to the 

 dorsal ends of the branchial bars. As shown in fig. 46, each 

 consists of a delicate excretory canal bent in the form of a 

 crook. The handle of the crook hangs down in the pocket- 

 shaped extension of the dorsal ccelom lying above, and leading 

 into the ccelomic canal of a primary gill-bar. The decurved 

 extremity of the crook is attached to the atrial wall just above 

 the adjoining tongue-bar, and here it opens into the atrial cavity 

 by a minute orifice situated on the end of a small papilla. 

 Internally the excretory canal does not open into the ccelom, 

 but its dorsal surface is produced into a number of short, 

 simple, or slightly branched closed diverticula, the rounded ends 

 of which are beset with a number of pin-shaped organs known 

 as solenocytes. Careful study with a very high power of the 

 microscope shows that each solenocyte has the following struc- 

 ture : The head of the pin is a knob of protoplasm containing 

 a nucleus. The shaft of the pin is an exceedingly delicate, 

 transparent, thin-walled tube, the cavity of which communicates 

 below with one of the branches of the excretory canal, but 

 has no opening into the ccelom. A long flagellum hangs 

 down from the terminal knob of protoplasm into the cavity of 

 the tube, and projects beyond the latter into the cavity of the 

 excretory canal. In life this flagellum is in constant move- 

 ment, and its flickering motion recalls the flame-cell of a 

 flat-worm, to which indeed the whole organ bears a close 

 resemblance. Carmine and other colouring matters intro- 

 duced into the blood system are taken up by the nephridia, 

 whose excretory function was proved by this method, and 

 it would appear that the excretory products pass by 

 osmosis through the thin walls of the tube of the soleno- 

 cyte, are propelled by the action of the flagellum into 

 the excretory canal, and pass thence by the excretory pore 

 into the atrial cavity, and finally by the atriopore to the 

 exterior. 



In addition to the tubules just described, there is a pair of 

 funnel-shaped structures lying in the dorsal ccelom in the 

 region of the twenty-seventh myotome, where the pharynx 

 passes into the intestine. The wide, funnel-shaped mouths 

 of these structures open into the atrium, and possibly their 

 inner narrow ends open into the dorsal ccelom. They are 



