10 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



concretion and is usually considered auditory. The canal of the stalk is 

 directly continuous with the gastro-vascular system. In the swollen 

 knob of the sensory club it forms an ampulla-like terminal expansion. 



As was pointed out by Glaus, the bottom of the sensory niche by 

 bottom is meant the vertical wall that separates the space of the niche 

 from the bell cavity is formed from the subumbrella only. This 

 arrangement of parts, apparently impossible for a structure so far 

 removed from the bell margin as the sensory niche, will be explained 

 more fully under the special topic of the vascular lamellae, or cathammal 

 plates. It is sufficient at this point to refer to Fig. 44, which shows the 

 shield-shaped area mapped out by a vascular lamella that connects the 

 endoderm of the stomach pocket with the ectoderm of the bottom of 

 the niche. By this the exumbrella is completely cut off from any part in 

 the formation of the bottom of the niche. Cross and vertical sections 

 through the niche (Figs. 39 and 37) help to a better understanding of 

 these relations. Since the base of the stalk of the sensory niche lies 

 within the ring of vascular lamella, the whole organ as well as the 

 bottom of the niche belongs to the subumbrella, and so in spite of its 

 position some distance upwards from the bell margin the sensory club is 

 very properly called a "marginal body" (Randkorper). 



The epithelium of the sensory niche consists entirely of the flattened 

 ectodermal surface layer common to the whole exumbrella. No differen- 

 tiation suggestive of nervous function in addition to that of the sensory 

 clubs can be discovered, although it would be quite natural to expect to 

 find something of the sort, as intimated by Glaus (78, p. 27). 



It is worth while to mention again the fact that the eyes are directed 

 inwardly toward the cavity of the bell. The larger and lower of the two 

 median eyes looks into the bell cavity horizontally ; the smaller upper 

 eye is turned upward toward the region of the proboscis. This is in the 

 normal pendant position of the sensory club. The stalk, however, is very 

 flexible, and a range of other positions of the sense organs is possible, 

 although nothing was observed to suggest that such positions were 

 within the control of the animal. The eyes evidently have as their chief 

 function to receive impressions of what is going on inside the bell, not 

 outside. Perhaps the strongly biconvex, almost spherical lenses of the 

 median eyes also point to a focus on near and small objects. 



5. The Bell Cavity and its Structures. In general, the bell cavity 

 repeats the external form of the bell, being almost cubical. In cross- 

 section it appears very nearly square with the angles in the interradii as 



