OBSERVATIOX OX FRESH-WATER POLYZOA. 115 



the outer and flic iiinei- cell-layers of the bod 3^- wall ; they run, 

 namely, immediately beneath the outer layer of the lophophoral ceil- 

 ino-. covered below by the lining epithelium. 



As mentioned above, the ({anodion is not a solid cell-mass as 

 has been described by nearly all investigators. On the contrary, it 

 contains a spacious ventricle, extending to the end of the arms, or 

 horns, as is diau^rammatically shown in fig. 30 a, h, c. PI. XIX. The 

 wall of the ventri(de is very thin and of an epithelial nature on all sides 

 except at the bottom somewhat on the anal side, ^vhere it is verv 

 thick, forming the ganglion sensu strict a, — a condition which reminds 

 us of the ïeleostian cerebrum. 



This thick portion is distinctly bounded from the thin epithelial 

 pai-t of the wall, and is well seen in the fre.sh state as a somewhat reddish 

 mass, with a slight constriction in the median plane of the polypide. 

 It is this part that Hyatt took for the ganglion which he describes as 

 comp(^sed of two lateral masses united by a verv tliick commissure. 

 It is no wonder that he overlooked the thin epithelial portion, since 

 this is hardly recognizable in surface views. As can readily be 

 imaodned by combininçf the three sections i>dven in fio-. 30, passino- 

 through the brain in different directions, the thick portion is a 

 transversely elongate rounded mass, with a transverse slit-like de- 

 pression, looking orally and upward. The whole mass is not of the 

 same structure throughout, but shows a differentiation into peripheral 

 and central portions. In the former, the nuclei (of ganglion-cells) 

 are densely crowded, while in the latter we see a faintly stained 

 granular mass (Punktsubstanz) containing only a few or no nuclei. 

 Tlie cell outline to each nucleus is not to be seen. 



The thin part of the wall of the ventricle differs in nothing from 

 ail ordinär}'' epithelium, being composed of a layer of flattened cells. 

 It is c«intiniious with the peri[)heral portion of the proper ganglionic 



