galloway: non-sexual reproduction in dero vaga. 133 



direction of the plane of the groove is shown in Figure 3. The depth of 

 the two infoldings is enhanced hy the outgrowing digitiform appendages 

 of the anterior zooid. When the depressions have reached a considerable 

 depth, they turn somewhat abruptly upward and backward on either side 

 of the digestive tract (Plate 3, Figs. 16, 17 ; Plate 4, Fig. 22, sul). The 

 ectodermal wall bounding this lumen applies itself to the lateral wall of 

 the old gut, but does not reach its mid-dorsal line. Posteriorly these in- 

 growths become continuous with the new pharyngeal wall described above, 

 and ultimately, after the separation of the zooids, their lumina, by the 

 breaking through of the tip of the invagination, are put in communication 

 with the lumen of the new pharynx (Fig. 19). This represents the con- 

 dition attained before the zooids separate ; the condition of the tissues 

 involved is shown in Figures 19-21. At the moment of separation, the 

 contraction of the semicircular muscles (Plate 3, Fig. 17; Plate 5, Fig. 

 26, mu. crc'ce.) and other muscles, both circular and longitudinal, in the 

 posterior zooid, pulls the whole prostomium downward, closing up the 

 communication of the body cavity with the outside world which other- 

 wise would result, bringing the aperture of the gut into a ventral, in- 

 stead of a terminal position. Simultaneously, the lateral fibres of the 

 dorsal muscle which lie inside the circumcesophageal commissure me- 

 chanically draw the ectoderm of the free anterior portion of the dorsal 

 wall inward and backward, under the brain, into the space left by the 

 failure of the lateral ectodermal ingrowths to meet in the mid-dorsal 

 line ; thus is formed the roof of the buccal cavity. A similar service in 

 the formation of the floor of the mouth is performed by the detached 

 constituents of the ventral muscle, in the mid-ventral line. 



The buccal cavity is thus lined with ectoderm from two sources : 

 (1) the floor and roof are produced by a simple infolding of already formed 

 ectoderm, unaccompanied by a process of growth ; (2) the sides, which 

 extend farther posteriorly, by the latero-ventral ingrowths. The deep 

 groove in the ectoderm (Plate 3, Fig. 16, sul. ; Plate 4, Fig. 23) 

 persists as a lateral furrow at either side of the mouth (Plate 5, Fig. 

 26, ivag. os. I.). 



When the zooids separate, and the lumina of the lateral ectodermal in- 

 vaginations become continuous with the lumen of the pharynx, there is 

 seen in this structure a pair of grooves (Plate 4, Fig. 18, sul.) which are 

 directly continuous with the ectodermal grooves of the mouth and buccal 

 cavity. By muscular contractions the old gut is loosened from its re- 

 maining points of attachment to the new, chiefly on the ventral aspect, 

 and the mouth and pharynx are functionally and structurally complete. 



