482 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



STALKED LARVA AFTER RUPTURE OF VESTIBULE, UP TO APPEARANCE OF 



RUDIMENTS OF ARMS. 



DESCRIPTION OF LARVA FROM FIFTH DAY TO SIXTH WEEK Al'TEU BUI'TURE OF EGO MEMBRANE. 



In its general form the larva is now characterized by the sharper differen- 

 tiation of the five-rayed calyx and the considerably elongated stem. In the calyx 

 the vestibule has broken through, showing the 25 tentacles in five groups of 5 

 each surrounding the mouth. 



At the time of the rupture of the vestibule (the fifth clay) the larvaB have 

 reached a length of about 1.25 mm. Five weeks after the liberation of the 

 embryo the larvae measure from the terminal stem plate to the outermost tips 

 of the tentacles 3.25 mm. ; about 2.50 mm. of this is included in the column. Aver- 

 age measurements are difficult to give, as the calyx can change its form to the 

 most extraordinary degree. 



ECTODERM. 



Ectoderm of body wall. While up to this time the pore formed the only con- 

 nection between the interior of the larva and the surrounding fluid, two new ones 

 now appear, through the rupture of the wall of the vestibule, and later through 

 the formation of the anus. 



In the stage last described the portion of the wall of the body covering the 

 vestibule had become considerably thinner. It becomes more and more flattened 

 and then sunken and funnel-like in the center, where the outer cuticular layer 

 on a very limited region merges with the pavement epithelium of the vestibular 

 roof, and a perforation appears. This rapidly enlarges to an elongate rounded 

 opening, which is large enough to permit the passage of the longest tentacle. 



Occasionally in conjunction with these processes a partial dissolution of mesen- 

 chyme cells can be noted, the resultant fluid being absorbed by the neighboring 

 tissues. 



In older larvae, of from 8 to 10 days, the opening into the vestibule has become 

 considerably altered. From the original central aperture five radial excisions run 

 outward between the five oral plates, so that the vestibular roof is divided into 

 five lappets. These excisions gradually extend to the neighborhood of the hydro- 

 coele ring, and the five lappets thus become the five triangular flaps containing the 

 five triangular oral plates so well known in the " prebrachial " or " cystid " stage. 



These five flaps are highly mobile; they may be turned inward, covering the 

 vestibule and the tentacles, or opened widely outward so that the whole calyx 

 appears disklike. This movement is rendered possible by a special musculature 

 in the hydroccele wall, in the ring canal as well as in the tentacles. 



Histologically the ectoderm has undergone only inconsiderable changes. The 

 protoplasmic cell bodies are somewhat more reduced than previously, the inter- 

 mediate substance, on the other hand, being somewhat more extensive. About 

 many nuclei the plasma is restricted to an extraordinarily thin layer, which is 

 either stretched out threadlike or spindlelike at right angles to the surface, or 



