AFFINITIES OF THE PELAGIC TUNICATES— BROOKS. L53 



twelve in the fourth, twelve in the fifth, fourteen in the sixth, and ten in the seventh. The 

 ascidiozooids in the first row are the largest, and there is a gradual decrease in size to the last 

 row, in which they arc no larger than the buds, /', that are carried by those in the first and second 

 rows. All are placed with their dorsal surfaces and brains toward the open end of the colony, 

 and their ventral surfaces and endostyles toward the closed end. The new buds arise at the 

 aboral end of the endostyle, on the ventral surface, as shown at /' in figure 8, and in figure 2. 



As the smallest and youngest aseidiozooids are nearest the open end, while they arise on the 

 surface of the zooid that is nearest the closed end, it is clear, from the figure, that a migration 

 must take place from the region where new buds arise to the region where they complete 

 their growth and development, and that this migration must, in some cases, be from one end of 

 the colony to the other. Each young ascidiozooid has two tubular processes, like those that are 

 shown in figure 1, on the region of the body that is nearest the open end of the colony. These 

 processes lengthen until they reach and enter into the diaphragm, where they are shown in figure :.'. 

 With a microscope they may be traced inward among the zooids and buds, although the com- 

 ponents of the colony are so crowded, and the tubes so delicate, that I have not been able to follow 

 any of them to the end, except the ones that end in the zooids in the row nearest the opening. 

 The number of tubes in the diaphragm, at the stage shown in figure 2, is nearly equal to the 

 number of zooids. but somewhat less, so that some of them must fail to reach the diaphragm, or 

 else degeneiate and disappear after they have reached it. The walls of the tubes are muscular, 

 and they are, no doubt, concerned in the opening of the diaphragm, and in the migration of the 

 zooids, although it is not probable that they bring about the migration by direct muscular con- 

 traction, since the distance that the zooids move seems to be too great to be brought about in this 

 way. It is more probable that the tubes become shortened by some structural change, which, 

 aided, it may be, by their muscular contractions, draws the new ascidiozooid into the region of 

 the colony where there is most room for it. In young: colonies there is most room at the open or 

 growing end, and the colony grows by the addition of new whorls of zooids at this end, and the 

 zooids are arranged in verticils. As the colony grows, new zooids become fitted into the spaces 

 between the old ones, and the verticillated arrangement which is so notable in the young colony 

 is no longer recognizable (fig. 4). 



