334 PROCEEDINGS OF THE AMERICAN ACADEMY. 



the definitive mouth is formed. As the Brachiolaria arms are visible 

 until a very late stage of metamorphosis, this method of determining 

 the principal plane of the body is very convenient, and at the same 

 time tolerably accurate. 



The aboral disk arises, as already known, as a thickened patch of 

 the ectoderm on the right side of the body, and extends also some dis- 

 tance on to the dorsal side ; but as metamorphosis progresses, it shifts 

 its position, so that it becomes more and more inclined to the sagittal 

 plane, and at the same time more nearly parallel to the crossplane, 

 i. e. a plane perpendicular to the chief axis of the body. The posi- 

 tion of the dorsal pore remains nearly, although not quite, constant. 



From the above it follows that the oral side of the adult is the ante- 

 rior, the aboral the posterior, the madreporic (interradius) the dorsal, 

 and the side opposite this the ventral, side of the larva. Right and left 

 sides are evident from what precedes. 



(2) As has been pai'tly suggested in the foregoing lines, both the 

 definitive mouth and anus are new formations. 



(3) Of the body cavity in the larva I distinguish, with Bury, four 

 portions, an anterior and a posterior on either side of the body. In 

 the fully developed Bipinnaria, these portions are all directly or indi- 

 rectly continuous with one another; but in the course of the Brachi- 

 olaria stage the right posterior portion is entirely cut off from the 

 remaining parts, and later, when the tentacles of the Bipinnaria have 

 been largely drawn in, another septum is formed anterior to the 

 first, while the left posterior portion also becomes constricted off from 

 the rest ; so that at this stage there are three separate cavities, viz. 

 (1) the right posterior, (2) the left posterior and the right middle 

 portions, which still form but one cavity, and (3) the right and left 

 anterior portions, which freely communicate with each other in the 

 Brachiolaria arms. The anterior cavity (3) persists in the adult as 

 the axial sinus and the water-vascular system. 



The right posterior cavity shifts its position hand in hand with the 

 aboral disk, and finally occupies the posterior end of the larva just be- 

 neath the disk. With the development of the arms of the star, it sends 

 out diverticula into them, and thus assumes the bibrachiate, o-radiate 

 star-shape which it has in the adult. 



(4) The formation of the water-vascular ring is not a mechanical 

 result of the breaking through of the adult mouth, for the ring exists 

 as such some time before the mouth is formed. 



(5) With Bury and MacBride I distinguish sharply two structures, 

 the pore-canal and the stone-canal. There is a stage when the pore- 



