MUSEUM OF COMPARATIVE ZOOLOGY. 63 



at first lie perpendicular to the roof of the colony (Fig. 86) gradually 

 come to lie parallel with it (Figs. 89 and 83). The oesophagus loses its 

 elongated, laterally compi'essed form, and becomes circular, and the gan- 

 glion lies just below the mouth-opening. Not until now, iu fact, can one 

 speak of a mouth. It was not at all formed synchronously with the anus. 

 To illustrate this process I have taken three different genera represent- 

 ing different stages. Similar stages could have been obtained from each 

 genus. By using three genera, the similarities as well as the dissimi- 

 larities of the process are indicated. Among other things, the larger 

 size of the polypide and shorter kamptoderm of the Ctenostome Flus- 

 trella (Fig. 89) is noticeable. 



Lastly, the coecum is formed as a wholly secondary differentiation of 

 the alimentary tract. This arises in some species relatively earlier than 

 in others ; thus it is better developed in Figure 86 than in the later 

 stage of Figure 83. 



The lining cells of the alimentary tract now rapidly undergo the dif- 

 ferentiations characteristic of the different regions. The most extreme 

 modification takes place in the lyharynx. In Cheilostomes the cells of 

 this region gradually become vacuolated, until finally very little stain- 

 able protoplasraa remains. The nucleus lies at the deep end of tlie cells. 

 A very peculiar modification of the cell walls takes place, in that they 

 become plainly perforated by holes through which the adjacent cells 

 are in communication (Fig. 85). It is iu a region similar to this that 

 the cells become cuticularized in Bowerbankia to form the so-called 

 gizzard. The pharyngeo-oesophageal region is also provided with a very 

 powerful musculature of circular muscles {inu., Figs. 85, 86). 



Concerning the origin of the muscles I have made very few stiidies. 

 The parieto-vaginal muscles seem to arise, as in Paludicella, from 

 around the neck of the polypide, and the retractors from the oral end 

 of the polypide bud (mu. ret., Fig. 89). 



The neck of the polypide sinks below the general level of the body 

 wall by an infolding of the latter, as described for Paludicella, and the 

 mass of columnar cells which passes down with it forms, I am confident, 

 the diaphragma of Nitsche ('71, p. 432), which is thus exactly com- 

 parable with the mass of cells around the atrial opening of Paludicella 

 in Figure 45, of. atr. (Plate V.). W\.ccording to this view, then, the dia- 

 phragma is not placed at about the middle of the kamptoderm, but at 

 its proximal end, and all that lies between it and the outer body wall — • 

 the non-evaginable portion — has been formed in the elongated neck, 

 exactly as the non-evaginable portion is formed in Phylactolsemata (see 



