62 ANATOMY OF AMFHIOXUS. 



the dorsal coelom. The posterior end of the tube also 

 opens into the ccelom, and between these two terminal 

 openings there is a variable number of other ccelotnic 

 openings, or funnels, as they are called, situated on the 

 dorsal side of the tubule, and opposite to that side which 

 carries the opening into the atrial chamber. The coelomic 

 funnels are placed at the ends of short upstanding projec- 

 tions from the main body of the tubule. On the ventral 

 side of the tubule, opposite in each case to a tongue-bar of 

 the pharynx, occurs the single opening into the atrial cav- 

 ity. The epithelium lining the tubule consists of cubical 

 ciliated cells. There is a thick bunch of cilia in connec- 

 tion with the atrial opening of the tubule. The curious 

 thread-like structures, carrying a round knob at their dis- 

 tal extremities, which radiate out from the ccelomic open- 

 ings, are specially modified cells belonging to the coelomic 

 epithelium, which are probably concerned in promoting 

 the excretory activity of the tubule, and are called by 

 Boveri, tliread-cells (Fadenzellen). 



The vascular supply and exact location of the nephridial 

 tubules (each tubule representing a nepliriduwi, according 

 to Lankester's nomenclature) are shown in Fig. 31. The 

 figure represents a piece of the upper wall of the pharynx, 

 cut out in such a way as to expose the inner wall of the 

 dorsal coelom. The cross is placed at the cut edge of the 

 double-layered membrane which separates the dorsal ccelom 

 from the atrial cavity. This cut edge can be traced from 

 side to side of the figure. The membrane is seen to be 

 continued down each primary gill-bar, in company with the 

 extension of the coelom, which runs down the primary bars 

 into the endostylar coelom as described above. On the 

 other hand, the membrane skips over the tongue-bars, so 

 that the atrial cavity is prolonged dorsalwards into a deep 



