MONOGRAPH OF THE EXISTING CRINOIDS. 497 



tissue of the body wall surrounding the parietal canal, so that it loses its indi- 

 viduality. The latter seems to be its ultimate fate in all the larvae. 



In young larvae the inner wall of the parietal cavity is always separated 

 from the ccelomic epithelium by mesenchyme. Little by little this is withdrawn, 

 and both walls gradually grow together into a single lamella. 



This lamella separating the ccelomic cavity and the parietal canal at first is 

 composed of two pavement epithelia, which can be differentiated in many larvae, 

 though in others there is only a thin partition with embedded nuclei to be seen, 

 which finally breaks down, so that the parietal cavity becomes merely a diver- 

 ticulum of the coelomic cavity. The intercommunication of these cavities appears 

 to become established in different larvae at widely different ages. 



The number of stone canals and pore openings through the calyx increases 

 considerably in the subsequent development. At the time of the separation from 

 the larval column five stone canals are present which open outwardly through 

 five pores as in the adult of the species of Rhizocrinus. Later the stone canals, 

 as well as the calyx pores, increase in number, without further correlation. As 

 Ludwig states that there are at least 30 stone canals in each interradius (in 

 Antedon mediten'anea) , while the number of calyx pores is approximately 1,500, 

 it necessarily follows that the later efferent vessels from the hydroccele can not 

 be formed in the same way as the first. 



As the result of muscular action the larvae assume very different forms, and 

 apparently the volume of the body changes also. This is probably the result 

 of changes in the volume of the water in the intestinal canal, which is open to 

 the exterior through the mouth, though partly due to the variation in the water 

 content of the parietal cavity, which opens to the exterior through the pore. 



MESENCHYME. 



Sacculi. After the rupture of the vestibule the first five sacculi enlarge 

 rapidly, becoming spherical or egg-shaped bodies, causing humplike swellings 

 in the cuticle, from which they are no longer separated by intermediate cells. 



Correlatively with the increase in size the structure of the sacculus changes. 

 An epithelial cortical Iaj 7 er separates itself from the inner mass and rapidly 

 becomes flattened, forming a membranelike pavement epithelium in which flat- 

 tened nuclei can be clearly made out inclosing the inner mass like a capsule. 

 There is no fusion between this and the surrounding mesenchyme. Often the 

 inner side of the capsule is composed of cells richer in plasma than elsewhere; 

 such a condition probably represents an intermediate developmental stage in 

 which the flattening has not yet been entirely completed. 



The cells which fill the capsule, originally showing the nucleus and cell 

 body clearly differentiated, after increasing by division become rapidly altered. 

 The form becomes rounded, and they unite in numerous groups, each of which 

 is composed of a number of individual elements, from two, three, or four to a 

 dozen or more. The size and form of each group varies with the number of 



