368 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



Mortensen believes that the eggs must break through the wall separating the 

 ovary from the marsupium. He could not determine definitely whether or not a 

 true opening exists, though in one instance this seemed to be the case. 



MAHSUPICM. 



In Notocrinus virilis (pi. 55, fig. 1351), according to Dr. Th. orMtensen, the 

 formation of the marsupium may be studied in the younger specimens. 



The first stage is a fairly deep imagination of the skin, widened at the 

 bottom. Further development consists simply of a widening of the sack, which 

 ultimately covers the whole distal side of the ovary. 



At first its lumen is rather narrow, but with the development of the embryos 

 it widens very considerably, so that it comes to occupy a considerably larger space 

 than the ovary. 



The walls of the marsupium are at first strongly ciliated, especially in the 

 middle of the bottom, where the epithelium is distinctly thickened. 



In the fully developed marsupium this thickening has disappeared, and the 

 ciliation also appears to have been lost. 



The opening of the marsupium remains unaltered, a deep, narrow pore with 

 a thick ciliated epithelium. This pore is situated on a fairly distinct papilla, 

 whicli stands on a low elevation like a small mammary gland. 



Since in the males the pore is not elevated, they are easily distinguished from 

 the females. 



In Thaumatometra nutrix (figs. 1232-1237, pi. 38) the marsupium is a pocket- 

 like groove on either side of the genital pinnules. A section through a genital 

 pinnule shows the empty marsupium as a deep, narrow-sided fold, the opening being 

 a very narrow slit. On the pinnules which carry large pentacrinoids the mar- 

 supium is widely opened. 



In Isomctra vivipara (fig. 1238, pi. 38), as described by Dr. K. A. Anderson, 

 the marsupium is always on the distal side of the ovary, from which it is separated 

 by a thin partition. 



When fully developed it is very large and contains numerous larva? in different 

 developmental stages. It occupies the region supported by the third, fourth, and 

 fifth pinnule segments, and to accommodate it these segments are expanded distally, 

 the third and fourth very considerably and the fifth slightly, giving the pinnules 

 a very asymmetrical appearance. 



The opening through which the fully developed larva; escape is slitlike, situ- 

 ated on the ventral side near the ambulacra! groove. 



In specimens which are not sexually mature the pinnules lack the marsupium 

 and are symmetrically developed. As the eggs begin to grow some small invagi- 

 nations appear on the ventral side of the pinnules, one of which soon becomes 

 larger than the others and forms the rudiment of the marsupium. As the eggs 

 become larger this imagination increases in extent until the marsupium is fully 

 formed. 



