456 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



With the growth of the whole body the digestive apparatus becomes extended ; 

 its walls become appreciably thinner, and its form alters in a remarkable way. 

 It becomes elongated in the direction of the major axis and at the same time 

 somewhat compressed dorso-ventrally. But sometimes it is enlarged dorso-ven- 

 trally, as well as elongated in the direction of the major axis, so as to appear 

 more or less baloonlike. As this is the usual form in the young attached animal, 

 this may be considered as premature development. 



Normally the dorsal wall is rather strongly convex, while the ventral wall 

 is concave, and in the concavity of the latter lies the water vessel. 



The dorsal wall is thick and is composed of a single layer of high prismatic 

 cells, of which the nuclei lie near the lumen. The ventral wall for the most 

 part is composed of a single layer of approximately cubical or low-prismatic cells 

 which carry the nuclei mostly in the inner half. 



Anteriorly the intestinal canal tapers rather abruptly and ends against the 

 parietal canal and the most anterior end of the right ccelomic sack. 



In rare cases there are found within the lumen in larvae just liberated a few 

 intrusive cells. Later these cells occur very extensively. 



Generally speaking, the relations as just described are only slightly altered 

 during a free-swimming period of two or three days. 



The gut swells out on all sides, eventually becoming globular, and as the 

 concavity of the ventral wall decreases individual cells detach themselves from 

 the wall and pass into the lumen. But this process is of limited extent, for at 

 the most Seeliger counted only 20 such cells in the oldest free-swimming larva, 

 while in many the gut lumen was entirely empty. 



In the most advanced larva- there appears approximately in the middle of 

 the ventral wall a small evagination directed toward the ectoderm. This elongates 

 through the center of the water vascular ring, this space being occupied merely 

 by a few mesenchyme cells. An extensive proliferation of ectoderm cells from 

 the lower side of the vestibular invagination grows out to meet it, though it is 

 usually later in the fixed larva, after the vestibule has entirely closed, that the 

 two unite. 



The dividing line between the ectodermic and mesodermic layers can be demon- 

 strated for a long time, and thus the esophagus is clearly seen to be a purely 

 ectodermal structure. 



Ccelome and Ohamtiered Organ. 



The shifting about of the two coelomic pockets originally lying right and 

 left, which began during the embryonal period, becomes much further advanced 

 in the free-swimming larva. 



The right ecrlomic sack has passed around the entire dorsal side of the intes- 

 tinal canal, posteriorly having almost entirely moved into a dorsal position, and 

 has permitted a process from the left sack to take part in bordering the gut on 

 the right side. Further anteriorly, however, it elongates on the right side of the 

 intestinal mass, covering the gut and water vessel, widely ventralward to the 



