PROCEEDINGS OF THE MALACOLOGIC.VL SOCIETY. 



NOTES ox THE ANATOMY OF IIAXLEYA ABYSSORUM, M. Sars. 



By R. H. BuRNE, B.A., 



Assistant in the Museum of the Boyal College of Surgeons of England. 



Read 8th November, 1895. 



PLATE II. 



1 . Ox THE RROBABLE PrESEXCE AND POSITION OF AN OsPHRADIUM. 



An osphradium, it need scai'cely be said, is the name given by 

 Professor Ray Lankester ' to a sense organ, presumably olfactory, 

 situated at the base of the gills in molluscs ; it reaches its greatest 

 development among the Prosobranchs, where it assumes a bipectinate 

 gill-like form constituting the well-known "false branchia." The 

 original discovery of this organ iu isolated instances, although in- 

 teresting, is practically of small moment, for it was not till 1881 that 

 it emerged from the obscurity of isolation, to rank as one of the most 

 important of mollusean organs. In that year Spengel- issued an 

 important paper on the relationships existing between the various 

 orders of the ]Mollusca, in Avhich he pointed out the constancy of 

 the occurrence and position of this olfactory organ in a great number 

 of instances, and from this constancy was led to infer the unity of 

 origin of the entire phylum. His wisdom in basing so great a 

 generalization upon such a foundation has been called in question, 

 as was almost inevitable ; but from the favour with which his views 

 are received in many quarters, especially, I believe, in England, we are 

 amply justified in regarding this sense organ as one possessing some- 

 what exceptional interest, not only for students of the Mollusca, but 

 for zoologists generally. 



Such being the case, one cannot but regret to find that among the 

 Chitons — molluscs that, on account of their many archaic character- 

 istics, are justly considered to possess a fundamental importance — the 

 exact position, and even the presence, of this organ are subjects still 

 enveloped in doubt. 



The following brief historical survey will display the present state 

 of our knowledge, or rather ignorance, upon this point. 



As far as I am aware, Spengol in 1881^ was the first to suggest 

 the presence of an osphradium in Chiton, but unfortunately it was 

 only a suggestion based upon superficial observation. As sundry 

 Chitons crawled up the sides of an aquarium, he noticed upon the 

 outer side of each gill a brownish patch, the position and general 

 appearance of which were strongly suggestive of an osphradium ; and 

 as such, Spengel was inclined to regard it ; but although he emphasizes 



1 Encycl. Brit., article " Molhisca." 



^ J. W. Spengel, "Die Geruchsorgane uud das Nervensystem der Mollusken " : 

 Zeitschr. Wiss. Zool, xxxv, 1881, p. 338. 

 •* Loc. cif. p. 356. 



