372 



R. T. GUN THEE. 



ectodermal thickenings of relatively enormous size. Though 

 at first lateral iu position, they soon become approximated, 

 and give rise to the '' ventral ganglion/' which, however, 

 never loses its primitive character of lying immediately 

 beneath the ectoderm. 



The pedal ganglia and their commissural loop do not exist 

 in any conspicuous form, nor should we expect to find them 

 iu the absence of a well-developed and muscular foot. It 

 might be possible to trace their homologues by making a 

 more minute study of the innervation of the hood; but at 

 present we merely suggest that cells corresponding to those 

 of MoUuscan pedal ganglia may be merged in the lateral 

 expansions of the cerebral ganglion already mentioned. 

 Von Jhering, on the other hand, believed that the 'Central 

 ganglion" consisted of visceral and pedal ganglia united. 



Pedal ganglia and nerves excepted, the Chaetognath 

 nervous system admits of a close comparison in point of 

 detail with that of Mollusca, but the particular type to which 

 it exhibits the closest affinity is that of Nautilus, as will be 

 seen by reference to the diagrams (text-fig. 6). 



A peculiar feature in Nautilus is the undivided nature of 

 the cerebral ganglion, which is also found in Cha3tognatha, 

 though unusual among Mollusca. The chief differences are 

 the absence of the pedal loop, the more centralised arrange- 

 ment of ganglionic cells on the visceral loop, the close 

 adhesion of the nervous system to the ectoderm and the 

 superficial nerve plexus. 



Sense Organs. 



The tactile organs of Cha}tognatha consist of isolated 

 neuro-cpithelial cells terminating iu tactile hairs, and of 

 groups of tactile bristles which belong to a type of sense 

 organ which is widely distributed among Mollusca. 



The "olfactory" (?) ring of ciliated cells and the area within 

 it (text-fig. 1, V.) is undoubtedly an organ of great importance. 



