Reprinted from THE ANATOMICAL RECORD, Vol. 11, No. 6 

 January, 1917 



46. The sensory potentialities of the nudibranch 'rhinophore.' * LESLIE 



B. AREY, Northwestern University Medical School. 



Nudibranch molltisca possess a pair of short, robust dorsal tenta- 

 cles which are commonly perfoliate or ringed and which may or may 

 not be retractile. These important looking tentacles have long been 

 designated 'rhinophores/ and it is tacitly assumed that they are in- 

 deed specialized olfactory organs. The presence in certain species 

 of long, more or less dorsally placed tentacles, in addition to the oral 

 tentacles and rhinophores, heightens the suspicion that the latter may 

 perhaps serve some particular sensory function. 



The sole experimental evidence upon which the assignment to the 

 rhinophores of an olfactory activity rests seems to be found in the 

 observations of Graber in 1877 (Biol. Centralbl., Bd. 8, No. 24, pp. 

 743-754). Graber brought oil of rose near the head of Chromodoris 

 elegans and observed the withdrawal of the rhinophores to be quicker 

 and more vigorous than that of the oral tentacles. He emphatically 

 states, however, that the post-branchial region is the most sensitive 

 part of the body. 



It would thus appear that the convenient term 'rhinophore' is of 

 dubious propriety. For this reason Bermtidian nudibranchs were 

 subjected to experimentation designed to test their sensory potentialities. 

 Unless otherwise stated the following account applies to Chromodoris 

 zebra Heilprin. 



Tactile stimulation. When a rhinophore is touched lightly with a 

 glass rod it is jerked back precipitately within its protecting collar. 

 The sensitivity of the rhinophore to gentle stimulation is astonishing 

 and the explosive type of response is, within wide limits, independent 

 of the strength of the stimulus. Fatigue comes on but slowly, responses 

 of somewhat diminished intensity being readily obtained after fifty 

 successive stimulations at ten-second intervals. 



The oral tentacles, gill plumes and the general body-surface all re- 

 spond to tactile stimulation. It is unsatisfactory to list the several 

 regions of the body in the order of their sensitivity, for the types of re- 

 sponses are not all comparable. It appears, however, that the so-called 

 rhinophore is the most sensitive part of the body and considerably 

 more so than the oral tentacles. 



Thermal stimulation. The head region and especially the oral 

 tentacles react distinctly to water at 40-50C. applied with a pipet. 

 The rhinophores, on the contrary, give faint and rather doubtful 

 responses except to temperatures as high as 50C. 



1 Contributions from the Bermuda Biological Station for Research, No. 52. 



