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[VOL. II. 



twenty-five specimens from distinctly tetramerous forms, hav- 

 ing short forks less than one-third the length of the entire 

 canal, or of such other variable aspects as to warrant their 

 inclusion under this head. PI. II, Figs. 6-9, and PL III, Figs. 

 15, will best illustrate this point. 



It now remains to consider somewhat more in detail the 

 individual variations exhibited by these several types. Direct- 

 ing attention first to those illustrated by the figures just cited, 

 it may be seen that the variation here seems to be in several 



FIG. 10. Symmetrically pentamerous specimen, but with the several series of 

 tentacles appearing at irregular intervals. 



directions, (i) Atrophy, as shown in PI. II, Figs. 4, 6, 8, and 9. 

 In all these the evidences of degeneration are quite clear. 

 First, in Figs. 6 and 9 there is the atrophy of the connection 

 of one of the canals with the gastric pouch and the correlated 

 reduction of the fourth gastric pouch and the further failure 

 of the obsolescent canal to develop its visual gonad, the merest 

 rudiments of which are apparent. Furthermore, in the same 

 figure there is shown a still further atrophy of a second canal, 

 which extends only about halfway to the margin, and corre- 

 lated with that fact is the associated imperfect development of 

 its gonad. A still further illustration of this degenerative 



