470 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



spherical form and becomes the stomach, while the other, resting on this with a 

 broad base, becomes a horizontal tube tapering to its blind end and forming the 

 blind gut. This latter quickly elongates, growing from right to left on the ventral 

 side, until at about the one hundredth hour it has reached approximately the 

 radius V. It runs in the now horizontal mesentery between the originally left 

 (oral) and right (aboral) ccelome in such a way that it only borders the oral coelome 

 with its posterior edge, while it protrudes into the aboral so that in section it 

 appears laterally and anteriorly surrounded by it. 



For a long time the gut, clear to the blind end, is filled with the mass of disin- 

 tegrated cells. 



CCELOMB AND ITS DBRIVATIVES (AXIAL AND CHAMBERED ORGANS). 



The alterations which take place in the two co3lomic pockets focus in changes 

 in position which are determined by the progress in the formation of the vestibule 

 and the intrusion of the closed vestibule into the hinder end of the larva. 



The turning about of the mesentery separating the two sections of the ccelome 

 takes place according to the same plan as that of the floor of the vestibule, so 

 that it passes over from the vertical to the horizontal plane and comes to lie at 

 right angles to the major axis. 



A longitudinal section through a 28-hour larva, just attached, shows the 

 coalomic sack only slightly altered. The mesentery has the shape of a horseshoe 

 and lies with both its side pieces, which are not quite the same, extending anteriorly 

 and somewhat ventrally, almost parallel with the floor of the vestibule. 



A cross section of a larva in which the vestibule is entirely closed cut on the 

 plane passing through the anterior end of the hydroccele shows similar relation- 

 ships; but the right ccelome has become more noticeably extended and on the 

 right side entirely covers the anterior tip of the left outwardly and reaches as 

 far as the vestibule. It also broadens out ventrally, inserting itself between the 

 gut and the tip of the left coelome, which advances toward the median line to 

 intrude itself between the hydroccele and gut. 



On the other hand, the right side piece of the left coelome extends itself on 

 the ventral side considerably farther toward the median line than does the left, 

 through which the circumstance, especially emphasized later, is already indicated 

 that the opening of the horseshoe does not lie in the median plane of radius V, 

 but is displaced toward the left. 



As the mesentery gradually acquires a horizontal position the last-mentioned 

 processes take on a greater and greater significance. Sections cut parallel to the 

 major axis illustrate this. 



A section lying near the median plane shows the left coelome cut through 

 dorsally and posteriorly between the gut and hydroccele, which have now become 

 considerably separated. In this space the left ccelome has extended itself as far 

 inward as the union between the gut and the floor of the vestibule. 



The sections to the right and left of this show that the left ccelome lies 

 entirely between the gut and hydroccele. The horseshoe-shaped form in cross 



