MUSEUM OF COMPAKATIVE ZOOLOGY. 25 



the body wall. In Figure 36 the line passing through the same points 

 makes an evident, but not very large, angle with the body wall. This 

 line has undergone, then, a slight change of position only. The axis' 

 of the anal tentacles lies in both cases nearly parallel to the body wall, 

 and so does the neural wall of the pharynx. The oral tentacles, on the 

 contrary, whose axes in the earlier stage are directed perpendicularly 

 to the body wall, lie in the later stage with their axes parallel to the 

 wall ; and the base of the lophophore, which in the earlier stage trended 

 at its oral end parallel, at its anal perpendicular to the body wall, in 

 the later lies throughout its whole extent in one plane perpendicular 

 to the body wall. The axes of the oral tentacles have rotated through 

 an angle of nearly 90° relatively to most of the other organs of the poly- 

 pide. The cause of this rotation must evidently be sought in unequal 

 growth in different parts of the polypide. A comparison of the length 

 of the kamptoderm on the anal side in Figures 25 and 7 indicates 

 that it has grown more in length than on the oral side. This excessive 

 growth would tend to rotate the line vlv. cr. — an. to a position perpen- 

 dicular to the body wall. Since this rotation has not occurred to so 

 great an extent as was to have been expected, we must look for a com- 

 pensating growth on the oral side of the polypide, between vlv. cr. and 

 the neck of the polypide, which shall be nearly equal to the excessive 

 growth of the anal kamptoderm, and which must be outside of the oral 

 kamptodem. These conditions of location are fulfilled only by the 

 oral wall of the oesophagus, and it is by change of position and growth 

 of this wall that the extension of the anal kamptoderm is nearly com- 

 pensated for on the oral side of the polypide. By this growth in the 

 wall of the oesophagus the oral part of the ring canal has been brought 

 to lie over the anal part, the sagittal diameter of the tentacular corona 

 has been reduced, and the compressed lophophore has^ been transformed 

 into a circular one. 



Concerning the number of tentacles, Dumortier et van Beneden ('50 

 p. 46) observe that in the adult there are ordinarily 16, although 

 individuals with 18 tentacles occur not infrequently, an observation 

 which Kraepelin ('87, pp. 98, 99) confirms. In addition to these num- 

 bers, I have found 15 and 17. The growth of the odd tentacle is quite 

 interesting. The sections reproduced in Figures 60-62 (Plate VI.) will 

 serve to illustrate a condition which I have quite frequently found in 

 a polypide with 17 tentacles. In this particular series there are only 

 15 tentacles. The successive sections abundantly demonstrate that 

 the odd tentacle (*) is anal in position, and that it is younger than any 



