and the Embryology of Autolytus cornutus. 401 



alimentary canal, as well as the young embryo, assuming a 

 more elongated shape ; and another constriction, placed 

 about the middle, is noticed, dividing the alimentary canal 

 into three regions. In the next stage (PI. X., fig. 4,) the 

 embryo is divided still more plainly by the constrictions, 

 the three regions of the alimentary canal are more marked, 

 the head with the eyes is more distinct, the triangular 

 shape has completely disappeared, and the mouth can be 

 seen as a small slit on the lower surface. In figure 5 three 

 rings or folds of the skin extend across the embryo ; the 

 anterior portion of the alimentary canal has become still 

 more narrow. Figure 6, which is figure 5, seen in pro- 

 file, shows the position of the alimentary canal near the 

 upper side, and the first appearance {A) of a slight swell- 

 ing between the two large eyes, the rudiment of the mid- 

 dle tentacle. In the following stage (fig. 7) the lateral 

 tentacles (A', A") appear as diminutive swellings in ad- 

 vance of the eyes. These soon outstrip in growth the 

 middle tentacle (fig. 8), which in its turn makes up what 

 it had lost, and the embryo takes the shape of (PI. X. 

 fig. 9) ; it has three short tentacles equally developed in 

 the anterior part of the head ; the anterior part of the ali- 

 mentary canal is very narrow, widening suddenly when it 

 opens into the main digestive tube. There are six rings, 

 the anterior ring being provided with a bunch of stiff 

 bristles, and the anal ring having on each side very small 

 anal cirri (E). The number of rings formed, till an embryo 

 reaches a certain state of development, does not seem to 

 be constant : in figure 10, PI. X., we have a larger number 

 of rings, though the tentacles of the head are not as ad- 

 vanced as they are in figure 9. The tentacles of the head 

 increase in length (fig. 12), two or three additional rings 

 are formed, and bristles are found in each ring except the 

 first and last. When it has reached this state (fig. 12) the 

 embryo does not increase in length until the appendages 



JOUKNAL B. S. N. II. 51 JULY, 1S62. 



