THE CEPHALOCORDA 187 



other openings placed at the ends of short offsets from the 

 dorsal side of the main tube. The single atrial opening is 

 placed on the ventral side of the horizontal part of the main 

 tube, and is furnished with a thick tuft of cilia. The interior 

 of the tube is ciliated, and in addition each ccelomic opening 

 is garnished with a number of curious thread-like structures, 

 each bearing a knob of protoplasm at its summit. These 

 so-called thread-cells are modifications of the coelomic epi- 

 thelium, and probably assist in some way in excretion. The 

 position of the excretory tubules in the plastic diagram, fig. 44, 

 is indicated at nph. The atrial openings of the tubules lie 

 close against the tongue-bars at the top of the pockets formed 

 by the so-called ligamentum denticulatum i.e. the two-layered 

 membrane which separates the dorsal ccelom form the atrial 

 cavity. This membrane is continued downwards for some 

 distance over the primary bars in connection with the ccelomic 

 canals contained in the latter, but it skips over the tongue- 

 bars, so that a number of pockets with the mouths facing 

 downwards are formed at the top of each primary cleft, and 

 into these pockets the excretory tubules open. The excretory 

 organs of Amphioxus are commonly called nephridia, and 

 they certainly present many analogies with the nephridia of 

 worms. But as we have restricted the name nephridium to 

 excretory tubes formed from epiblastic ingrowths, and as we 

 know nothing of the development of the excretory system in 

 Amphioxus, it is undesirable to make use of a term having 

 a limited signification. The excretory tubes of higher verte- 

 brates are formed as ccelomic outgrowths, and are therefore 

 described as peritoneal funnels. We might expect that the 

 excretory tubes of Amphioxus are formed in a similar manner, 

 but it is better, while we know nothing definite about the 

 subject, to refer to them simply as excretory tubes. That 

 they are excretory is proved by their taking up and excreting 

 particles of carmine that have been introduced into the 

 system. 



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 



