AN UNDESCRIBED ACRANIATE. 227 



The true mouth is at the centre of a transverse velum provided 

 with a strong sphincter muscle and a series of oral tentacles that 

 are not, apparently, quite as numerous as in the European lancelet 

 though having the same arrangement. 



The capacious pre-oral chamber bears upon its side wall the 

 so-called " Raderorgan," consisting of three large ridges of specially 

 thickened, ciliated epithelium, passing forward on each lateral wall 

 (Figs. 1, 2) to end in rounded lobes in the posterior half of the 

 chamber above the basal part of the pre-oral hood. These ridges 

 might be regarded as one continuous ridge folded back and forth 

 into three chief loops with smaller lobes interpolated between them 

 at the posterior end. All the ridges continue posteriorly as a cili- 

 ated area leading to the velum and out on it towards its central 

 aperture, the mouth. An important part of the structure is a special 

 ridge found only upon the right side, high up towards the noto- 

 chord, so that it appears median from a side view (Fig. 1). In 

 sections " anterior to the above paired ridges (Fig. 13) this right, 

 azygos ridge continues to end only at the extreme anterior angle of 

 the pre-oral chamber. Here it is associated with the conspicuous 

 pre-oral pit found on the right side in the position shown in Fig. 

 1 2, but continues as a special ciliated ridge even anterior to the 

 pit (Fig. 18) as well as posteriorly to join the other ridges at the 

 velum. 



The pit has the form of a glandular tube, somewhat bent, opening 

 out into the pre-oral chamber in the azygos ridge and histologically 

 divided into two regions. The terminal part is ciliated, the internal 

 closed end has an epithelium of what appear to be gland cells 

 casting out a coagulable substance into the lumen. As already 

 stated the two regions of the organ behave differently when the 

 animal is fed with carmine. 



That the pit has either an excretory function or, more probably, 

 may serve to secrete slime to aid in entangling food particles to 

 be swept back to the mouth, is an idea suggested by its action 

 towards carmine and strengthened by its anatomical relations to a 

 large blood vessel or space. This vessel passes along close beneath 

 the azygos ridge for its whole length (Figs. 12, 13) and is the 

 " aortic arch " of Langerhans (28) as figured by Ralph (29) and 

 described by Lankester (23) and recently called a " glomus " by van 



