388 LEFEVRE. [Vou. XIV. 
a and 6, many mesodermal cells (w/.c.) attach themselves to 
the inner surface of the ectodermal depression, become greatly 
elongated, and are eventually transformed into muscle fibers. 
In Botryllus, according to Pizon (22), it is the branchial wall 
which thickens and evaginates to fuse with the ectoderm, while 
the latter plays but a small part in the production of the orifice. 
The process, as it occurs in Perophora viridis, is quite similar 
to that described by Kowalewsky (11) for Phallusia, and by 
Van Beneden and Julin (33) for Clavelina. 
The cloacal orifice is formed in exactly the same manner, by 
the union of an ectodermal invagination with the dorsal wall of 
the cloaca (Pl X XIX, Fig.s5, andy Pl XOCXT) Wiss 23,/2:c7-0.), 
It will not be necessary to speak here of the various appen- 
dages and ciliated growths of the pharynx which arise later, — 
namely, the tentacles, papillae, languets, dorsal lamina, and 
peripharyngeal bands, — as these are merely differentiations of 
the pharyngeal epithelium, and have been sufficiently described 
by numerous authors. 
The Digestive Tract. 
Some time before the displacement of the inner vesicle is 
completed, and when the folds which will cut off the peri- 
branchial cavity are not very deep, the wall of the inner vesicle 
high up on the left side at the extreme posterior end becomes 
- much thickened, and soon evaginates to produce a little blind 
pouch, the rudiment of the digestive tract (Pl. XXX, Fig. 15, 
gt.r.). This lateral diverticulum grows out as a tube, which at 
once bends sharply downwards and forwards, while, as the shift- 
ing of the vesicle continues, its opening into the latter is carried 
up nearer and nearer the mid-dorsal line, where it will ulti- 
mately come to lie. The tube soon turns abruptly on itself to 
form a close U, and, now growing upward along the outer wall 
of the left peribranchial sac until it reaches the dorsal surface, 
finally bends directly forward, and stops short at the posterior 
wall of the cloacal cavity. At this point the distal extremity 
of the tube fuses with the cloacal wall, an opening breaks 
through, and the anus is established. The differentiation into 
