THE BNTEEOPNEUSTA. 5 



whicli projects into the proboscis cavity. For reasons given 

 later in this paper I propose to speak of this structure as the 

 noto chord {Nch.). The general appearance of the animal at 

 the time when the second gill-slit appears, is shown in fig. 1. 

 The animal is drawn as seen from the actual left side and 

 displays the increased flexion of the body on its ventral 

 surface. 



The second gill-slit is shown as a small circular pore. 



It will also be observed that that part of the body which lay 

 between the two grooves constituting the middle segment of 

 the body has now assumed an altered shape. At Stage H is 

 found little more than a circular ridge on the body separating 

 the proboscis from the trunk, while in fig. 1 (two gill-slits) it 

 forms a kind of phlange enveloping the base of the proboscis. 

 This change in shape is due to the operation of several causes, 

 which are of great importance in interpreting the processes by 

 which the final form of the adult is reached. 



In the first place, after the formation of the mouth as a pore 

 on the ventral surface, the constriction by which the proboscis 

 is segmented oflF becomes deeper and deeper, until at last it is 

 only attached by the exceedingly slender stalk shown in figs. 3 

 and 3. As a consequence of this process coupled with a 

 forward growth of the ventral lip of the collar, the mouth 

 comes to be directed anteriorly instead of ventrally (cp. figs. 

 7, 46 and 57). By this process the anterior phlange of the 

 collar acquires the relation shown in fig. 1, et seq. In 

 addition to these changes a most important structure is first 

 formed at this time, namely, the cavity, which from the rela- 

 tions which it afterwards possesses I shall speak of as the 

 atrial cavity. 



In the later conditions of Stage H the body is perceptibly 

 wider in the region of the body immediately anterior to the 

 gill-slits than it is behind them. This increase in width, which 

 is still very slightly marked, is due to a circular thickening 

 which passes all round the animal, being most developed at the 

 sides. By the time of the appearance of the second pair of 

 gill-slits this thickening has considerably increased, and in the 



