126 
MEMOIRS OF THE RATIONAL ACADEMY OF SCIENCES. 
the blind forms have the longer olfactory rods, and that they are developed so as to compensate 
for the loss of vision. 
Si nous comparons entre elles les baguettes de nos Amphipodes d’eau douee, c’est-h-dire celles des Gammarus 
pulex, fluviatilis et puteanus et celles de I’Asellus aguaticus et de la forme provenant des puits, nous verrons que ces 
baguettes n’atteignent pas chez toutes ces espbces le meme dtiveloppement; celles des Gammarus pulex et fluviatilis 
et celles de 1 ’Asellus aguaticus sont courtes, atteignant it peine le tiers de la longueur de l’article suivant, tandis que 
les baguettes du Gammarus puteanus et de l’Asellus des puits fgalent en longueur et mo me ddpassent l’article suivant. 
(Je me sers ici de 1’article suivant comme mesure comparative, vu que tous les articles des tigelles de ces esptces sont h 
peu prfes dgaus entre eux et que les baguettes, fixges it l’angle des articles, sont dirigfes parallfelement it l’article qui suit.) 
Nous remarquerons ensuite que les baguettes courtes appartiennent aux espbces pourvues d’organes visuels et 
que les baguettes longues, que nous considdrerons comme trfes ddveloppdes, appartiennent aux deux espfeces habitant 
les eanx souterraines et qui sont aveugles. 
Comparant d’une manifere gdndrale l’importance du sens visuel et olfactif dans l’dconomie animale, nous 
verrons que l’un et 1’autre sont d’une ndcessitd dgale, ou que si l’un est atrophid, l’autre par contre s’est ddveloppd, 
s’est perfectionnd de manidre h pouvoir remplacer l’organe ddfectueux (l. c., p. 12). 
Afterwards Leydig, and also Fries, confirmed this view, and the former remarks in his Unter- 
suchungen, etc., 1883: 
In two Crustacea of our native dark fauna it has been for some time possible to indicate such a change in rela¬ 
tions.* The olfactory rods on the larger flagellum of the upper antennse in the eyeless Gammarus puteanus are more 
developed than in species provided with eyes. Short, and scarcely reaching in Gammarus pulex and Gammarus fluvia¬ 
tilis a third of the length of one flabellar-joint, they are in Gammarus puteanus nearly of the length of the succeed¬ 
ing flabellar joint. On the smaller antennae of the blind Asellus cavaticus the olfactory rods are likewise much more 
developed than in Asellus aguaticus provided with eyes. 
The olfactory rods in the species of Orconectes are longer, according to Faxon, than in normal 
Cambari and Astaci. Leydig, who was the first to examine these organs, claimed that the olfactory 
rods in these Crustacea were more numerously developed than in the normally eyed forms, and 
that this was a compensation for the loss of sight. Leydig counted on the outer flagellum of the 
first pair of antenna; thirty six joints. Of the olfactory rods—for they are rod-like rather than like 
teeth ( Riechzapfen , as Leydig calls them)- all occur on the distal end of the flagellum; two occur 
on the anterior end of the fifteenth joint, while on the succeeding joint there is a third one. On 
the distal ends of most of the succeeding joints there are four rods and in the middle three, or 
seven in all, but towards the end of the flagellum the number of rods again diminishes to two and 
finally to one rod, their greatest development being in the second third of the flagellum. 
A single olfactory rod which Leydig figures (Taf. Ill, fig. 27) consists of a thick walled stalk 
and a more delicate terminal tube, which is thicker than the stalk. The rod is perforate at the 
thickened end by a narrow opening which connects with the hollow interior. The substance of 
the interior of the stalk appears to contain coarse granules, that of the terminal part fine ones. 
Although Leydig does not say so, it is evident, in the light of the more recent studies of Hauser 
and Kraepelin, that this hollow olfactory rod, perforate at the end, is fundamentally homologous 
with the olfactory teeth of insects. Our own observations on the rods of Orconectes hamulatus con¬ 
firm the accuracy of Leydig’s description of those of 0. pellucidus (see Pi. XXII, figs. 6, 6a, 6b, 6c). 
Prof. R. Ramsay Wright (Amer. Naturalist, xviii, 272) examined these rods in Gambarus 
propinquus, finding the external branch of the first antenna composed of eighteen or nineteen 
joints. The distal nine of these alone bear olfactory cones, and only five of them (the eleventh, 
twelfth, thirteenth, fourteenth, and fifteenth) have the full number of eight on each joint; hence 
he concludes with Leydig that the olfactory rods “ are present to a much greater number than in 
allied forms possessed of sight.” 
There however seems a greater variation than suspected in the number of joints to the outer 
flagellum, and it is the greater length of the rods in the blind species which indicates greater 
olfactory powers. In his “Revision,” etc., Professor Faxon makes the following statement: 
I have examined several specimens of C. propinquus with reference to this point, and find that the number of 
segments in the external flagellum of the antennule may be as high as thirty-five, fifteen or sixteen of which may 
* As regards the latest statements, we might quote Leydig, Ueber Amphipoden und Isopoden. Zeits. f. wiss. 
Zool., xxx, Suppl., 1878. S. Fries, Mittheiluugen aus dem Gebiete der Dunkelfauna. Zool. Anzeiger, 1879. Fries 
states: On the short upper antennae of Asellus cavaticus not only do the-number of joints vary, but also that of the 
olfactory rods. Sometimes only the two penultimate joints, sometimes three or four joints, are provided With these 
rods; in one instance I counted six such rods. Rougemont records five to seven long olfactory rods. The rods 
themselves are in A. cavaticus very considerably longer than those of A. aguaticus , 
