468 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



the cavity of the vestibule, each divide themselves into three parts. Thus are 

 formed 15 tentacles which grow rapidly to cylindrical processes and receive an 

 ectodermal investment from the base of the vestibule. 



Simultaneously with the migration of the vestibule the hydrocoele ring has 

 assumed a position almost exactly at right angles to the major axis. It has 

 become noticeably wider, but still remains narrower than the diameter of the 

 anterior (basal) vestibular border, so that the tentacles rise directly within the 

 latter. 



The arrangement of the tentacles in five groups of three each corresponds to 

 the radial structure of the adult. The five central tentacles lie in radii I-V and 

 farther outwardly than the others, since they occupy the rounded angles of the 

 pentagon. The groups of tentacles are separated in the interradial planes. As 

 yet the insertions of the tentacles are not quite in the same plane, so that in a 

 transverse section they are cut through at different heights. 



The ectoderm of the tentacles is composed throughout of rather high cylin- 

 drical cells disposed in a single layer. On the inner side it is somewhat higher 

 than elsewhere, and here, since the nuclei lie at different heights in adjoining 

 cells, it often appears to be arranged in several layers. 



On sections through the bases of the tentacle groups the ectoderm on the 

 inner side of the three tentacles appears fused together, that on the outer wall 

 being well defined. 



In slightly older larvae the esophagus is composed of elongated, almost wand- 

 shaped cells, most of which have the nuclei in the middle. 



In a somewhat more advanced stage the number of tentacles has risen to 25, 

 for two interradial tentacles have formed between each of the groups of three. 

 These are noticeably shorter than the others and are united at the base by a bridge- 

 like elevation of the vestibule wall. Though similarly arising from the hydro- 

 coele, they are situated more toward the central axis of the animal than the other 

 tentacles, so that they stand on the periphery of a circle of smaller diameter. 

 Their structure is identical with that of the older tentacles. 



Before the rupture of the vestibule the tentacles grow to a considerable length, 

 their ends bending inward under the roof of the vestibule toward the center. In 

 part the extension of the tentacles takes place at the expense of their diameter, 

 which becomes progressively less and less. 



INTE.STINAL CANAL. 



Soon after fixation the gut appears much extended and balloon-like, a little 

 flattened toward the hydrocoele, and through the center of the latter connected 

 with the vestibule. 



In the older free-swimming larvae individual entoderm cells can be seen to 

 detach themselves from approximately the center of the epithelial lining of the 

 flattened wall and to enter the lumen, though usually this process first takes place 

 in the fixed larva. Apparently none of these cells are given off from the dorsal 

 wall. In the nourishing juices in the cavity of the gut these cells multiply with 



