MORPHOLOGY OF MELIBE 519 



to a sense of taste and smell at all, or even to a common 

 chemical sense, but rather to designate the particular senses 

 in question a general chemical sense. Moreover, Arey working 

 experimentally on several nudibranchs, Chromodoris 

 zebra, Facelina goslingi, Elysia crispa, and 

 Fiona marina, found that there was nothing in the tests 

 which he applied to these animals that connected the rhino- 

 phores with olfaction, with the exception perhaps of Facelina 

 whose non-retractile rhinophores react by a lashing withdrawal 

 and more vigorously than the oral tentacles when stimulated 

 by oil of pennyroyal, carbon bisulphide, and anilin oil. All 

 these forms, however, responded to tactile stimuli. Like 

 Copeland, he found that the general body-surface also responded 

 to chemical stimuli. Crozier and Arey (1919 : 301) elaborated 

 on this by stating that in Chromodoris the rhinophores 

 and the oral tentacles are in a general way the parts most sensi- 

 tive to chemical stimuli. Again, as regards the rhinophores, 

 these authors (1 91 9 : 278-81 ) found that while C h r o m o d o r i s 

 zebra may creep in an entirely normal fashion after the 

 rhinophores have been removed, it loses its power of orientation 

 to the water current. In other words, these authors claim 

 that to currents of adequate velocity the nudibranchs are 

 negatively rheotropic and that the rhinophores are the prime 

 receptive organs for this kind of reaction. However, as I have 

 pointed out elsewhere (Agersborg, 1922 a: 432, 439), the 

 dorsal tentacles (rhinophores) of Hermissenda opales- 

 cens (Cooper),^ do not seem to have a rheotropic function, 

 because specimens with one or both of the dorsal tentacles 

 removed oriented as easily and moved against the current as 

 did the normal individuals. It did not seem to make any 

 difference whether the dorsal tentacles were present or not. 

 At any rate, the rhinophores or dorsal tentacles do not seem 

 to be ' rheotropic 'in Hermissenda. Copeland, Arey, and 

 Crozier found that the types with which they w^orked were 

 more sensitive around the anterior than elsewhere on the 

 body. Such a specialization of the integuments is also the case 



^ The correct name is Hermissenda crassicornis Eschscholtz. 

 Vide C. H. O'Donoghue (1922), ' Nautilus ', 35 : 74-7. 



