BUDDING PROCESSES OF CEPHALODISCUS DODEGALOPHUS. 521 



three ventral nerve-tracts which are still very prominent. In the adult they are 

 greatly reduced. In some respects they remind one of the large ventral nerve-mass 

 in young Sagitta. 



The muscles are now quite distinct, and in fig. 63, which is through the base of 

 the pedicle, they are seen to extend round dorsally. 



Development of the Plumes and Pinnae. 



The plumes arise throughout in pairs. They first make their appearance as a 

 papilla, which elongates to a finger-shaped process (fig. 69), the distal extremity of 

 which becomes slightly swollen (fig. 70), and then bulbous (fig. 71). The epithelium 

 of this bulbous extremity then becomes modified to form the eyes. The cuticle of 

 certain of the epithelial cells becomes thickened (fig. 75), and soon the thickening 

 protrudes into the cavity of the cell as a lens-like body (figs. 76 and 77). Later it is 

 detached from the cuticle and lies freely in the protoplasm. Here it becomes rounded 

 off to form the lens. 



Immediately below the eye a pair of small processes arise laterally (fig. 72), and 

 grow rapidly to form the paired pinnae. The other pinnae arise as a double row in 

 succession downwards (figs. 73 and 74). 



Each plume is at first circular in outline, but at an early stage the ectoderm of one 

 side is pushed in to form a groove (fig. 40), so that the outline in section is horseshoe- 

 shaped. 



The order of development of the plumes is peculiar. The first pair arise close to [ ^ 

 the middle line, with their groove facing towards the dorsal surface of the buccal ,r 

 shield (figs. 40 and 64). The second pair arise outside these (figs. 49 and 65), and are 

 at first in the same relation to the buccal shield. 



In the course of development, however, both pairs commence to rotate upon their 

 own axis, and also to move in a posterior direction away from the buccal shield. 



Thus, in fig. 49, the plumes have rotated about 90°, or a little more in the case of 

 the first pair. The grooves are not evident at this level, but the nerve tracts are 

 always at the opposite side to the grooves. The third pair of plumes arise outside, and 

 at the base of the second pair, and they behave subsequently in a similar manner to 

 these. Thus the three pairs of plumes come to lie a little way back from the buccal 

 shield, and with their grooves and pinnae facing away from this organ. Fig. 27 shows 

 this arrangement, and also the rudiment on the left side, of one of the fourth pair. 



The three last pairs of plumes are later in development, and differ slightly in their 

 growth from the three first pairs. The fourth arise almost parallel to and nearer the 

 buccal shield than the third, whilst the fifth appear first between the second and the 

 buccal shield, and lastly, the sixth arise just in front of the first pair. These points are 

 illustrated in figs. 66, 67, and 68. These three pairs develop grooves facing the buccal 

 shield, as in the case of the others, but they do not rotate. 



