252 



E. W. MACBRIDB. 



Fig. 89 is a section of a larva of Stage D; both hydrocoeles 

 are well developed— the right, in fact, better than the left ; the 



Ihy/ 



Fig. 2. 



Fig. 3. 



■PC 



right hydrocojle appears on the left side of the figure, since by 

 an oversight the section was drawn from the wrong aspect. It 

 took me some time in this larva to determine which side was 

 which, but the right hydrocoele is rather more dorsally situated, 

 and opens by only a narrow slit into the anterior ccelom. It is 

 also curved somewhat differently, the most posterior lobe being 

 No. 4, not No. 3, as on the left side. Fig. 90 shows a most 

 remarkable variation. We see a pore opening directly from the 

 hydrocoele to the exterior. If, as I shall attempt to show later, 

 the anterior ccelom may be compared to the proboscis cavity of 

 Balanoglossus, and the two hydrocoeles to the collar cavities of 

 that animal, we see that what we may term a collar-pore may 

 arise as a variation. Figs. 91 — 94 are sections taken from a 

 larva of Stage G. Its only abnormality is that in connection 

 with the right hydrocoele, which is of normal character, a 

 second pore-canal and stone-canal are developed. Fig. 92 

 should show the opening of the second stone-canal into 

 the hydrocoele ; fig. 93 the opening of conjoined pore-canal 

 and stone-canal (compare woodcut 3) into the axial sinus. 

 Fig. 91 shows that the two pore-canals unite, to open by a 

 common median pore. The above are not by any means all 

 the variations observed, but they are sufficiently typical to in- 

 dicate their nature. 



