194 Messrs. Hancock and Embleton on the Anatomy o/Eolis. 



sheath, which is long, and into which the tentacle is quite retrac- 

 tile, is garnished around its extremity by a circle of arborescent 

 filaments, by which the organ even, when extended, is to a 

 great degree protected from injurious contact with surrounding 

 objects. 



Again, as if the laminated disposition of the tentacle were not 

 sufficient for the purpose of the Antiopa splendida, PI. VI. fig. 10, 

 we have these organs, a «, standing out from the sides of a me- 

 dian crest, h, which is elevated above the surrounding skin, and 

 crowned by a series of pinnate laminae. That this median crest 

 is really a part of the olfactory organ, an addition to its com- 

 plexity, is proved by the attendant modification of the nervous 

 element, which is as follows. There is directly in front of and 

 in contact with the median cerebral a pair of small ganglia, c c, 

 each of which gives ofi* two branches, one of which, </, goes to 

 the tentacle, and the other, e, much thicker, goes to one half of 

 the median crest. 



We could easily adduce other examples from the Doridida, 

 if others were required, to show the importance and the spe- 

 ciality of these organs in the Nudibranchiata, but those we have 

 brought forward seem enough for this purpose. Another cir- 

 cumstance bearing upon the special nature of these tentacles, 

 and noticed by Joshua Alder, Esq., one of the authors of this 

 paper, in a communication made to the British Association 

 at the Cork meeting, is that the cilia on their surface vibrate in 

 a direction contrary to that of those on the surface of the 

 branchial papillae. On these the cilia move constantly from the 

 body towards the extremity of the papilla ; on those they act from 

 the point of the tentacle towards the body ; thus, in the former 

 case, the water which has served for respiration is drawn from 

 the body and thrown ofi" from the apices of the papillse, whilst in 

 the latter the fluid which we may suppose to contain odorous 

 particles or qualities is attracted to the end of the tentacle, and 

 made to pass down over the entire surface, and then thus to act 

 upon the sentient nerve within. 



Now it is a constant occurrence in the higher animals that the 

 fluid to be tested by the olfactory organ is always brought to the 

 nerve, and made to pass over the sensitive surface in the majo- 

 rity of instances by means of the agency of inspiration. In fishes 

 however in which the nasal cavity is shut ofi" from the mouth and 

 throat, another agency than that of respiration is required; the 

 olfactory plates however are freely supplied with cilia, and these 

 probably act a part analogous to those of the laminated tentacles 

 of Eolis. But the dorsal tentacles are not only, according to our 

 view, important and special organs, but they are, further, organs 

 of smell. Their laminated structure is one evidence of this. The 



