STIMULATIONS IN NUDIBRANCHIATE MOLLUSKS 425 



DISCUSSION 



The following data were collected from experimental studies 

 of Hermissenda opalescens Cooper, Dendronotus giganteus 

 O'Donoghue, and Melibe leonina Gould. 



Type 1 — Hermissenda opalescens Cooper 



This species has been accurately described by O'Donoghue 

 ('21) ; it is, however, necessary to give a brief account of certain 

 morphological structures because of the variance in the nomen- 

 clature employed by many writers relative to certain points. 

 Gasteropods have commonly one or two pairs of tentacles on the 

 head. The anterior pair is frequently spoken of as oral tentacles 

 or buccal or inferior (Fischer) , vordere Fiihler (Glaus and Grob- 

 ben, '10), or Mundtentakeln (Lang and Hescheler), and the 

 posterior as dorsal tentacles, hintere Fiihler (Glaus and Grobben), 

 or Kopf tentakeln (Lang and Hescheler) , or rhinophores by other 

 writers, notably by Bergh. In some species (Melibe Rang, 

 Dendronotus Aid. & Hanc.) the oral tentacles may be lacking, 

 as a distinct pair, or may consist of one or more rows of varied 

 sized cirrhi around the fringe of the hood (Kjerschow Agersborg, 

 '19, '21, '21 a, '22), and in that way the oral tentacles sometimes 

 may not be readily recognized. The dorsal tentacles may also be 

 modified so as to make them at first sight quite indistinguishable 

 from the papillae (dorsal cerata), as in Dendronotus. In this 

 paper I will treat of the rhinophores as dorsal tentacles (not 

 ' head tentacles' (Kopf tentakeln) , as they are sometimes situated 

 on the neck, e.g., Hermissenda, etc.), and the cerata as papillae 

 (Kjerschow Agersborg, '22). The anterior or buccal tentacles 

 may most appropriately be called oral tentacles not only because 

 of their proximity to the mouth, but also because of the common 

 usage of the term in this connection. 



The oral tentacles (O'Donoghue,' 21) in Hermissenda opalescens 

 consists of one pair, situated anterolaterally on the head. They 

 are lanceolate, tapering gently to a point. In a specimen which 

 measured 40 mm. from the anterior end to the tip of the foot, 

 they were 12 mm. or a little less than twice the length of the dorsal 



