136 BULLETIN OF THE 



proliferation at intervals in the upper wall of the ring canal ; thus a 

 projection is formed at each of these points, which constantly elongates 

 to form the tentacle. Figuies 70 and 60 (I'late VII.) are longitudinal 

 sections of two later stages in the development of tentacles. The inner 

 layer, ex. (Fig. 70), becomes gradually thinner as the tentacle grows 

 older, and its cells finally become thiead-like (Fig. G9, ex.). 



Figure 81 (Plate IX.) shows the arrangement of tlio tentacles about the 

 mouth and over the ganglion in a young polyjiide. The supraganghonic 

 part of the ring canal is cut tangentially just beliind the e])istome (cu)/. 

 crc.'"). I have often noticed that, in polypidcs of about the age of that 

 of Figure 77, or older, certain of tlie nuclei seen in a cross section of a 

 tentacle stain more deeply than the others. These nuclei are usually 

 two or three in number on each of the lateral surfaces of the tentacles. 

 The}' arc evident in Figure 81. I do not know what this difference in 

 staining properties signifies. Yigclius ('84, p. 38, Fig. 23) descrilios 

 and figures a condition of the nuclei in Flustra, as seen on cross-section, 

 which is similar to that just described. The deeply staining nuclei in 

 Flustra lie on the inner fiice of the tentacle, are larger than the otliers, 

 and belong to cells which possess no cilia. 



Kitsche ('71, p. 43) described the development of the tentacles in 

 Flustra as thouoh tluv wei'e derived cxclusivclv from tiie inner lavcrs of 

 the bud ; but Kepiachofi" ('7^)^ pp. 138, 139, '75^ j). 1-52) showed that in 

 Cheilostomes both cell layers ()f the bud took part in their formation, 

 and he figures an early stage which is quite similar to my Figure 70. 



4. Development of the Lophoplioric Nervea. — It has long been known 

 that a large nerve passes along the middle of the upper wall of each 

 lophophore arm, connecting proximally with the corresponding side of 

 the ganglion. Xo observations have been made, so far as I know, upon 

 the oriirin of this organ. Fvidently there are, a priori, two possibilities. 

 Either (1) the lophophoric nerve is formed by a direct outgrowth of 

 the ganglion, or (2) it arises in place from the inner layer of the bud, 

 which, since it here forms the outer layer of the lopho])h()ric pocket, is 

 the same as that fnm which the ganglion itself is constructed. By a 

 careful stndv of this nerve in many stages of development, and from sec- 

 tions in (lifTci-ent directions, I have come to tlie conclusion that it arises 

 as an outgrowth of the walls of the ganglion, and that it penetrates 

 between the outer and inner lavers of the arm. 



The facts which have led me to this conclusion are these. First, dur- 

 ing the formation of the brain, soon after its lumen is cut ofi^ from its 

 connection ^vith the atrium, its cells begin to divide rapidly (Plate V. 



