MUSEUM OF COMPAKATIVE ZOOLOGY. 129 



IV. Organogeny. 



1. Development of the Ring Canal. — Nitsche ('75, p. 358) describes 

 the ring canal as a furrow arising from the opening of each of the lopho- 

 phoric pockets, and running towards the oral side of the bud. In a later 

 stage, both layers become deeply implicated in this furrow, and the 

 ring canal is completed by a growing together of the edges of the 

 furrow. 



Braem ('89 b , p. 679) merely states that he cannot fully agree with 

 Nitsche's description of the formation of the ring canal. 



As a result of my own studies on this subject, I have readied the 

 conclusion that the circumoral branch of the ring canal makes its first 

 appearance in the median plane in the oral region at about the time 

 that the depressions of the lophophoric pockets are first indicated. The 

 formation of both organs is preceded by a preliminary thickening of the 

 inner layer of the bud (Plate IV. Fig. 26, br. lopk., and Plate III. Fig. 

 17, can. crc). It is only later, after the lophophoric pockets have at- 

 tained considerable depth, that the groove of the incipient "ring canal " 

 appears continuously on the side of the polypide, extending from the 

 pre-oral region to the lophophoric pockets (Plate IV. Figs. 33, 35, 37, 

 can. crc). 



As indicated in the successive stages of Figures 18 and 19, Plate III., 

 the thickening of the inner layer anterior to the mouth is followed by a 

 fold at this point involving both layers. The fold is deepest in the pre- 

 oral part of the median plane, and becomes shallower as it proceeds pos- 

 teriorly. Finally, the outer-layer cells of the lips of the fold approach 

 each other and fuse, thus forming a true canal (Plate IV. Fig. 33, can. 

 crc). Kraepelin ('87, p. 57, Figs. 72, 73, qb.) asserts that this canal 

 does not communicate at its neural ends with the ccenocoel, but that it 

 is always closed by a strong " Querbriicke " connecting the " Kampto- 

 derm" with the alimentary tract. By making sections of the colony 

 parallel to the sole, dozens of individuals are cut through the entire 

 length of the circumoral ring canal. Although I have examined many 

 individuals cut in this way, I have never succeeded in finding in Crista- 

 tella this closing "Querbriicke"; but in both young and old specimens, 

 sections nearly corresponding to Kraepelin's Figure 72 show a perfectly 

 uninterrupted semicircular space surrounding the oesophagus, and open- 

 ing freely into the ccenocoel on each side of the brain (Plate IX. Fig. 78, 

 can. crc). I must therefore conclude that in Cristatella the fluids of 

 the cavities of the circumoral branch of the ring canal, and therefore 



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