THE EARLY DEVELOPMENT OP AMPHIOXUS. 603 



(fig. 21, a). Oil tlic left side the front section of the ventral 

 collar cavity seems to become solid, and from it are apparently 

 derived the true oral tentacles, so well described by Lankester, 

 and the muscles moving them. " Hatschek's nephridium " 

 has now become a horizontally placed tube, the openings of 

 which into the first myotome (collar cavity) on the left and 

 into the gut are easily seen (fig. 21, b and c). It is curious 

 that this internal opening completely escaped the observation 

 of both Van Wyhe and Hatschek ; it is perfectly easy to see in 

 any good series of sections through a specimen preserved in 

 osmic acid. Hatschek (4) states that this nephridium persists 

 into the adult. I have, however, been able to find no trace 

 of it after the metamorphosis. 



The ventral extensions of tlie collar cavities appear in the 

 larva to reach some distance behind the last gill-slit formed. 

 In any series of sections in this region we get quite similar 

 appearances to those represented in figs. 20, a, b, and c. The 

 natural conclusion to be drawn from this is that behind the 

 first gill-slit, as the gut grows and produces new gill-slits, the 

 collar cavities grow pari passu. 



The coelomic cavities in the somites we saw at their time of 

 formation to be exceedingly minute; in some cases it would be 

 more correct to say that there was no cavity at all, but only a 

 radiate arrangement of the cells round a virtual cavity. Such 

 a state of things might in the minds of some suggest that 

 there was something to be said for LwofF's position that 

 Amphioxus is not an enterocoelous animal, since its coelom 

 could not be traced into continuity with the alimentary cavity. 

 Such a position, however, appears to me to be quite untenable, 

 even apart from the fact that the collar cavity is at one time 

 in open and obvious communication with the enteric space. 

 What determines whether an animal is to be regarded as 

 enterocoelous or not is whether or not evidence is forthcoming 

 to show that the walls of the body-cavity have been derived 

 from those of the alimentary canal by a process of folding, for 

 it is the walls only to which we can attribute an objective 

 existence ; the presence or absence of a cavity at any moment 



