34 BULLETIN OF THE 



One cannot refrain from noticing the similarity in the relations of the 

 "under lip" of Rotifers (Zelinka, '91, Taf. III. Fig. 55) and the epistome 

 of Bryozoa (cf. also the foot of Mollusks). 



Zelinka has also shown (p. 397) that in Callidina at an early stage 

 two lateral folds appear on the ventral side of the embryo, enclosing be- 

 tween them the mouth and under lip. These folds extend along about 

 one half of the length of the embryo. Later they become relatively 

 shorter, and finally form the lobes of the wheel organ. They have also 

 precisely the position of the lophophoric ridges of Ectoprocta at an early 

 stage, as I have figured it ('91, Fig. 25), lying on each side of the mouth 

 and ganglion. They have also the same relations as the ridges from which 

 the gill filaments arise in Lamellibranchs. Lankester's ('74, p. 80) view 

 of the homology of the tentacles of Bryozoa and gill filaments of Lamelli- 

 branchs is thus strengthened. 



Finally, there is between Eotifers and Endoprocta a striking similarity 

 in the position of anus and urogenital ducts, which in Urnatella, as in 

 Rotifers, open into a common cloaca on the suboesophageal aspect be- 

 tween mouth and anus. Such a resemblance is especially striking in 

 footless genera like Asplanchna (which, however, has no anus, cf. Ma- 

 sius, '91, fig. 1) and Hertwigia (L. Plate, '85, Fig. 7), — genera resembling 

 more nearly the ancestral form, since the foot, lying behind the anus, 

 must be considered as a secondarily produced appendage. 



To sum up : The embryological as well as the anatomical evidence 

 seems to sustain the view that Bryozoa are closely related to Rotifers, 

 the two groups having sprung from an ancestor which was common to 

 them and Mollusks also ; that after the Rotifer stem had branched off, 

 the common Mollusco-Bryozoan stem produced tentacles on the lateral 

 ridges ; that the two groups then soon separated, the Mollusca to un- 

 dergo an extensive and complicated development, the Bryozoa to remain 

 at a low level. The chief changes which the Bryozoa have experienced 

 are (1) the acquiring of a body-cavity through the relative decrease in 

 amount of the mesenchyme, that which remains forming an epithelium 

 (Phylactolaemata) ; (2) the loss (T) of the protonephridia and sexual 

 ducts in Ectoprocta ; (3) the loss of the epistome (Gymnolaemata) ; 

 (4) the loss of the preoral ganglion ; (5) the acquiring of a coecum (Ec- 

 toprocta) ; (6) the multiplication of methods of reproduction, by regen- 

 eration, by budding (without and with stock-foi'mation), by division of 

 stocks, and by statoblasts. 



Cambridge, September 27, 1892. 



