COMPARATIVE ANATOMY OF THE MAMMALIAN ORGAN OF JACOBSON. 237 



Echidna. — In Echidna the organ is considerably simpler than in Ornithorhynchus, 

 and it is uncomplicated by any remarkable cartilaginous developments. W. N. Parker 

 has studied the structure of the organ and its relations in the young animal, while the 

 adult condition has been described by myself. 



The most striking point of difference of the organ in Echidna from that of Ornitho- 

 rhynchus is the absence of any anterior development. The nasal floor cartilage is com- 

 paratively simple, and, in the adult, but feebly developed, and quite distinct from the 

 alinasal. Immediately in front of the region of the naso-palatine canal, the nasal floor 

 cartilage is a small cartilaginous plate which abuts against the base of the nasal septum 

 by its thickened inner edge, and becomes rapidly thinned away on passing outwards, 

 ouly forming a support to the inner third of the nasal floor. On passing a little back- 

 wards the thickened inner part becomes divided off from the slender outer part by the 

 naso-palatine canal passing upwards (figs. 4 and 5). At first the inner part is irregularly 

 square shaped, but a little farther back it becomes hollowed out on the outer side for the 

 reception of the duct of Jacobson's organ, as it opens into the naso-palatine canal (fig. 5). 

 On passing still farther back this inner cartilage becomes very distinctly " C "-shaped, 

 with the anterior end of Jacobson's organ in its concavity. The cartilage is about 

 uniform in thickness, but at the outer end of the upper part of the " C" there is a very 

 distinct thickening. A very little beyond this plane the lower part of the "C" joins 

 with the thickened outer rim, and Jacobson's organ becomes completely enclosed in 

 cartilage. The naso-palatine canal receives the duct of Jacobson anterior to its opening 

 into the nasal cavity, so that the organ only communicates with the nasal cavity in- 

 directly by the upper part of the canal. Fig. 6 represents a section through the 

 posterior wall of the naso-palatine canal. To the outer side of the cartilage of Jacobson 

 the thickened portion will be observed (rud.t.); this is found, on examining the suc- 

 ceeding sections, to be continuous with the turbinal, and is, no doubt, the remnant of a 

 turbinal which once extended to the front of the organ as in Ornithorhynchus. The 

 organ itself at this plane is found on section to have the ordinary mammalian kidney 

 shape. The outer part of the nasal floor cartilage is still seen as a small inconspicuous 

 fragment. A short distance behind the plane of fig. 5 the organ is met with in its full 

 development, and assumes an appearance which it retains to near its posterior end. On 

 section it is found to be nearly circular with the outer wall so completely invaginated 

 as to leave very little lumen in the organ. It is completely surrounded by cartilage, 

 and the invaginated wall is supported by a flat turbinal plate of cartilage passing in- 

 wards from the outer wall of the capsule. The capsules of each side rest on a flat 

 cartilaginous plate, which passes right across from the one side to the other, and com- 

 pletes what would otherwise be a little gap in the lower wall of the cartilage of Jacobson. 

 This cartilaginous plate is exactly comparable with the similar plate in Ornithorhynchus, 

 and is developed from the small outer portions of the nasal-floor cartilage, which become 

 greatly enlarged, and pass inwards. 



On comparing the structure of the organ and its relations in Echidna with that in 



