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 wsophagus loses its 
elongated, laterally compressed form, and becomes circular, and the gan- 
glion lies just below the mouth-opening. Not until now, in fact, can one 
speak ofa 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 pharynw. In Cheilostomes the cells of 
this region gradually become vacuolated, until finally very little stain- 
able protoplasma remains, The nucleus lies at the deep end of the 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 in a region similar to this that 
the cells become cuticularized in Bowerbankia to form the so-called 
gizzard. The pharyngeo-wsophageal region is also provided with a very 
powerful musculature of circular muscles (mu, Figs. 85, 86). 
Concerning the origin of the muscles 1 have made very few studies. 
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.). According 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 Phylactolemata (see 
