NOTES ON HISTOLOGY OF LINGULA ANATINA. 23 



the ileo-parietal band and a portion of the nephridium from a 

 transverse section through a young Lingula (5.5 mm. in shell 

 length) is shown. Here we see at once that the ileo-parietal band 

 is nothing but a folding of the visceral layer of the mesoblast, 

 which covers the nephridia as well as the alimentary canal. 

 Moreover in the figure we can distinguish two kinds of cells 

 which constitute the ileo-parietal band : the one with vesicular 

 nuclei, and the other with compact nuclei. The former are those 

 which have already acquired the character of ova, while the 

 latter kind of cells are those which still remain as ordinary 

 epithelial cells. The supporting substance between the two layers 

 of the ileo-parietal band is not as yet formed. 



In young Lingula of 9-14 mm. shell length the layers of 

 the ileo-parietal band bearing the young ova are subjected to a 

 high degree of folding, so that in sections they give a dendritic 

 appearance. 



In the female which has almost attained maturity, the ovary 

 appears as a conspicuous mass of cells of a darker brown than 

 the liver, filling up the main part of the body cavity, and pro- 

 truding even into the " Erker." The ova (PL IL, Fig. 27) are 

 prism-shaped, pentagonal or hexagonal in section, and slightly 

 rounded out on their free surfaces. They measure 60 t x in 

 diameter and 90 n in height. Their cytoplasm (PI. II., Fig. 28) 

 contains minute yolk granules uniformly distributed. In larger 

 ovarian ova vacuoles are found scattered throughout the peripheral 

 portion. The vacuoles apparently result from the dissolving away 

 of the yolk granules. 



The nucleus, enclosed by a definite membrane, lies always 

 near the free surface : a portion which must bear some relation 

 to the metabolic function of the ovum (PI. IL, Fig. 27). The 



