MUSEUM OF COMPARATIVE ZOOLOGY. 129 



around the base of the knob. If we watch the growth of this collar, 

 which is the outer differentiated layer of the ectoderm, it will be found 

 to gradually grow down around the sacculus until it has almost com- 

 pletely enclosed it, leaving, however, an opening at the distal pole of the 

 knob, through which the end of the sacculus, or certain appendages to 

 the extremity of this organ, project. (PI. I. figs. 4, 5.) 



Meanwhile, the sacculus has passed through certain changes, the most 

 important of which is a coiling up of itself within the envelope of the 

 involucrum, and the formation at its extremity, where it projects through 

 the opening of the involucrum, of certain appendages of a provisional 

 nature. The earliest condition of the sacculus is simply the terminal 

 transparent part of the flask-shaped body already mentioned. It is 

 now a complicated organ armed with lasso cells, and with its walls 

 highly colored. The provisional structures at the distal end of the 

 sacculus (PL I. figs. 4, 5, 6, 8) are mentioned and figured by most of 

 those who have studied the young knob of PJiysophora. They have 

 been seen in both species, but do not appear to exist in the fully de- 

 veloped form of the knob, either in P. hi/drostatica or P. Philippi. The 

 accompanying growth of another part of the young knob is destined to 

 change materially the appearance of the whole, as well as the relative 

 development of the parts. This change takes place contemporaneously 

 with the enclosure of the sacculus by the involucrum, and the appear- 

 ance of those provisional terminal filaments which I have already men- 

 tioned. The alterations of form to which I refer are as follows. The 

 proximal or basal part of the spherical-shaped expansion of the young 

 knob enlarges on one side, and in such a manner that the knob as a 

 whole assumes an asymmetrical shape. (PI. I. fig. 4.) This want of 

 symmetry is brought about by an unequal growth in the two sides of 

 the basal part of the knob itself. In a still more developed stage of the 

 same structure the inequality in growth has gone still further, and the 

 enlargement lengthens and extends along the side of the sacculus, now 

 coiled on itself, carrying with it the former place of attachment of the 

 sacculus, which is to be found at the opposite pole from its former junc- 

 tion. (PL I. figs. 5, 6.) 



Meanwhile the knob is approaching its fully-grown form, and the ter- 

 minal filaments become absorbed ; the opening at the distal pole of the 

 involucrum closes or is very much reduced in size, and the enlargement 

 in the spherical cavity, which earlier gave the asymmetrical form to the 

 whole knob, appears as a simple tube following down along the side of 

 the knob from the pedicel to the place of attachment of the sacculus, at 



