THE AMERICAN 
MONTHLY 
MICROSCOPICAL JOURNAL. 
Vor. VIIL APREL, -1687).5 No. 4. 
Spengel’s olfactory organ, or osphradium in crepidula. 
By HENRY LESLIE OSBORN.* 
In the various writings upon the anatomy of the gastropoda, prior to 1880, 
there will be found references to an organ which is variously called the 
‘rudimentary gill,’ ‘ accessory gill,’ etc. It is figured in very many. of the 
drawings of the anatomy of prosobranchs, and in general has somewhat the 
appearance of the gill, but is smaller and lies parallel to the gill and upon its 
left. A number of conjectures respecting its physiology are to be found. 
Williams, in his article upon the Mechanism of Respiration,t conjectures 
that it is of use as a color gland. Kieferstein, in the article, Mollusca of 
Bronn’s Klassen und Ordnungen, teaches that it is the aborted gill of the left 
side of the body, and that conjecture, which is a very obvious one, on super- 
ficial examination, was the general view as to the use of this problematical 
structure, and is indicated in the use of the term ‘ rudimentary gill’ by Claus 
in his Zoology (vol. 2, p. 48). This idea was based upon a merely superficial 
view of the relations of the parts, and led to a conclusion as to the homology 
of the organ, which was erroneous, viz :—that it was the gill of the left side 
crowded out and aborted by the translocation of the functional gill from the 
right side. That the functional gill of Ctenobranchs is the one of the right 
side, though actually found on the left side, is hardly doubtful at present in 
view of our knowledge of the twist in the visceral nerves of the group ; but that 
the so-called rudimentary gill is not a gill at all has been well shown by 
Spengel.t{ 
The structure of this organ has never been very thoroughly elucidated ; 
indeed there is nothing published which gives a complete description of its 
fine structure in any species. Spengel, in the paper already alluded to, treats 
of the nervous system and its relation to the area in question, but does not 
treat of the histological structure of the organ itself. It is seen in its most 
typical form in Strombus and Buccinum. Here it is seen as a median rod, 
carrying transveral flaps. It runs parallel with the gill upon its left, but is 
shorter and roughly resembles that organ in shape. Sections across the organ 
show that the central longitudinal axis contains a nerve-trunk, from which 
fibres are distributed to the transverse leaves, the final destination of which 
fibres has never been determined except conjecturally. They appear, how- 
ever, to terminate in cells upon the surface of the flap or leaf as their active 
end-cells. 
Spengel in his paper shows that close to the gill, or gills, there exists an 
area of modified epithelium, and in close relation with this a nerve which in 
every case he finds to be derived from the supra-intestinal ganglion. He finds 
* Read before the Indiana Academy of Sciences Dec. 20, 1886. 
+ Ann. and Mag. Nat. Hist., 1854. 
{ Geruchsorgan der Mollusken.—Zeitschr, f, w, Zool., 35, p. 333. 
