28 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



Figs. 50 and 51 represent sections of a gill, taken in the planes 

 indicated in Fig. 36. The chitinous rod is cut in one, but the plane 

 of the other lies below it. These sections show the gill to be rather 

 evenly ciliated. Later stages, after the lobes have elongated, show 

 the cilia to be confined to the side next to the mantle. The third 

 lobe is formed from the ventral lobe, which broadens and constricts 

 in the manner detailed above. Other lobes are formed in a similar 

 manner. 



The alimentary canal elongates, bends ventrally, and the stomach 

 enlarges. In from two to four days after the test is cast, the digestive 

 glands reappear. At first they do not have very definite walls, but 

 they gradually become well defined, and it is noticeable that the large 

 scattered cells disappear at a corresponding time. The two pouches 

 are no longer symmetrical, the left being prolonged posteriorly fills 

 up the space between the dorsal end of the stomach and the intestine, 

 Figs. 36 and 66. The greater development of the left digestive gland, 

 at this early date, possibly affects the future position of the intestine, 

 a loop of which comes to lie on the right side of the animal, very 

 near the shell. 



About the 25th day, a ridge, corresponding in position to the outer 

 labial palps, appears as an outgrowth from the body wall. The palps 

 did not attain any considerable development in any of the embryos 

 reared from eggs. 



The ganglia are all distinct in whole mounts as well as in sections. 

 At first the cerebral ganglia are much larger than either of the 

 others, Fig. 33, but the pedal ganglia grow rapidly, and soon come to 

 equal if not to exceed them in size, Fig. 36. No division into cerebral 

 and pleural ganglia has been observed. 



GENERAL REMARKS. 

 ANATOMY. 



Since Mitsukuri's 8 paper appeared in October, 1881, much has been 

 written on the anatomy of members of this group. Pelseneer 11 and 

 others have called attention to certain characters in support of the 

 supposition that the group is primitive. 



Some of these characters do not seem to hold good with Yoldia. 

 Thus on page 274 in speaking of the primitive characters of the group 

 Pelseneer 11 includes among others the following : 



