of homology is the mode of innervation of an organ. Spengel reasoned that the homologies of these organs could be best established by a comparative study of their modes of innervation—in other words, by discovering their relations to other organs known to be correlated in definite ways among themselves. In this way he succeeded in proving their morphological identity, although the belief that they are olfactory organs is based simply on the morphological fact that they invariably occupy a certain position in relation to the respiratory organs, and not on any physiological data.
He demonstrated the general occurrence of this particular kind of organ in the prosobranch gastropods, inferred that it ought to occur among the opisthobranchs, and succeeded in demonstrating its presence in the division of tectibranchs. He had already in his possession the hypothesis that the organ is one belonging to the mollusca as a whole and drew from it the deduction that it ought to be present in the lamellibranchs, among which it had not been hitherto known. He said: "The position in which such a one would have to be sought was clearly enough indicated to me by my observations on the gastropods. It would have to be in the neighborhood of one of the ganglia of the visceral commissure." Trusting this definite anticipation, he looked for the olfactory organ and found it in Arca Noæ, the first mussel he opened for the purpose. In this species the organ is characterized by pigment, which made its recognition easy. In other species that he examined the pigment is absent, and had he first opened one of these, he might have had a long and possibly fruitless hunt for the organ. This well illustrates how important a part chance frequently plays even in deductive investigation. It is interesting to note how the deduction might have remained unverified and possibly have beenand yet have been a true one.
The organ typically consists of thickened epithelium innervated from a ganglion underlying it. Theory required the presence of a ganglion under the olfactory organ of lamellibranchs, but there was apparently only a strong nerve, which had hitherto been universally interpreted as the "gill nerve." Histological examination proved it to be an elongated ganglion inserted on the nerve between its origin and its ending in the gill. Here again, a deduction led to a discovery and the correction of what had seemed for years to be a settled fact.
Spengel had shown, in his study of other groups, that the nerve on which the olfactory ganglion lies arises from the vis-
- Die Geruchsorgane und das Nervensystem der Mollusken. Zeitschrift für wissenschaftliche Zoologie (April 22, 1881), vol. xxxv, pp. 333-383.