s 
88 Observations upon the Habits, Structure and | February, 
folds, and, about the time of the formation of the last cleft, the 
mouth aperture begins to change its position, moving forward very 
slowly toward the cartilaginous welt and the ciliated pit. Accom- 
panying this forward movement of the mouth the anterior pharyn- 
geal clefts become pushed over toward the right side of the 
pharynx, and toa slight extent under the right fold ; the posterior 
part of the ciliated pit grows backward, burrowing into the tissue 
toward the cartilaginous welt; and the cartilaginous welt broad- 
ens throughout its upper portion, the upper posterior limb grow- 
ing backward towards the mouth, and a slit or opening forms 
through its center so as to transform it into an irregular pear- 
shaped ring of cartilage placed obliquely across the anterior por- 
tion of the central cavity. Fig. 5, Pl. 1, represents a diagram- 
matic cross-section of the body at this stage of growth. The 
section is made transversely through the mouth and one of the 
pharyngeal clefts, as at æ, Fig. 7, Pl.u. The mouth opening, 
with its teeth-like processes, is seen upon the right side; the 
mouth welt with zfs processes below it; the pharyngeal cleft 
pushed just to the left of the median ventral line ; and still fur- 
ther to the left a slight indication of the anterior portion of the 
right body fold. The mouth and pharyngeal apertures open 
directly into the digestive tract. 
The forward movement of the mouth, and the backward move- 
ment of the ciliated pit, continue until the anterior edge of the 
mouth aperture is close to the posterior limb of the cartilaginous 
ring, andthe posterior portion of the ciliated pit has burrowed 
back to the opening in the ring, when a split takes place in the 
body wall extending from the mouth opening forward over the 
the edge of the cartilaginous welt and through into the ciliated 
pit, Fig. 2, Pl. 11, in which only the edge of the ciliated pit is in- 
_ tral dotted lines of the figure. This split now deepens or extends _ 
to the center of the pit, forming a passage-way into the digestive 
_ tract through the cartilaginous ring; the mouth aperture and split — 
in the body wall gradually close; the bottom of the ciliated pit — 
becomes attached to the edge of the cartilaginous ring, which is 
already attached to the walls of the anterior extremity of the 
pharynx, and the food passes into the digestive tract from this 
~ time forward by way of the newly opened aperture in the bottom — 
ok the ciliated e This i is a marked advance in dopao 
dicated, the central, deeper portion being back towards the cen- 
A Na A e tay ee 
