Te MORGAN. [Vot. IX. 
plate, and a blind diverticulum extends on each side posteriorly 
along the lateral walls of the digestive tract (not seen in 
figure). The opening to the exterior of this body-cavity is 
shown in the figure, at the small pore lying a little to the left 
of the mid-dorsal line. 
The proboscis-vesicle has also enlarged. It now lies just 
above the tubular portion of the body-cavity leading to the 
dorsal pore. It is applied in this region to the body-cavity on 
its dorsal side, but is not indicated in the figure. See Fig. 32, 
Pl. IV. The proboscis-vesicle is, as pointed out by Met- 
schinkoff, contractile. I have seen it beating rythmically at 
this stage. A long diastole alternates with a slow systole. In 
what portions of its wall the contraction takes place was not 
determined. 
The second and third pairs of body-cavities were not seen in 
the living animal, although sections now show that they are 
present. 
During this period of its life the larva may be considered to 
have reached the climax of its pelagic existence, and it begins 
to undergo changes preparatory to its transformation into the 
young worm, having an entirely different arrangement of 
organs suited to new conditions of life. The change is so 
marked, indeed, that it might be spoken of as a metamor- 
phosis. 
The next stage drawn is shown in Fig. 4, Pl. I, but very 
many intermediate stages between this and the last have been 
studied. The outline of the larva is completely changed. Four 
prominent alterations may be noted. First, a decrease in size, 
as indicated in the relative sizes of the two drawings. Secondly, 
about the middle of the larva one notices a deep constriction 
lying at the level of the horizontal limb of the anterior ciliated 
band. Thirdly, the posterior plate has bulged outwards and 
backwards, so that instead of a flat plate it now presents a 
convex surface to the exterior. 
In the fourth place, it will be noticed that the digestive 
tract, which is still connected at (or near) the center of the 
posterior plate, has been pulled (?) backwards during this period, 
