614 THE OKOAN OF JACOBSOX IX :\IARSUPIALS, 



end of the tube. Two large nerves lie at the upper end of the 

 tul^e, and on the inner side ai^e two or three moderately hirge 

 veins. There is, however, no hilar plexus as in Phascolarctus. 



Co-"\]PAHATIVE OBSERVATIONS. 



From the examination of Jacobson's organ in the various types 

 of Marsupials, it will be noticed that although thei'e are many 

 variations of details, tlie same general plan is followed in all; 

 though the habits of the different animals vary greatl}^ and with 

 the habits are very distinct diffei'ences of tooth structure; though 

 some of the animals are nocturnal and others lovers of the light, 

 some gregarious and others solitary ; all possess moderately 

 de^■eloped organs of Jacobson, and in all have we the one main 

 type of structure followed. 8tudies in Eutherian forms lead to 

 the same conclusions, viz., that the type of organ does not \ary 

 with the habits, but remains constant throughout large groups of 

 apparently not very nearly related animals. For example, we 

 have one type in such dissimilar forms as the Ox, Sheep, Horse, 

 Dog, Cat, and Hedgehog, but quite a different type in the 

 Rodents. From this constancy of type followed by the organ it 

 is manifest that it must be a very valuable factor in the classifi- 

 cation of groups — apparently of more importance than even the 

 dentition. 



Before considering the morphological importance of the different 

 varieties in the Marsupialia, a few general observations may be 

 well. In Mammals generally it would seem that the organ is 

 best developed in small forms, and that in animals which ha^■e 

 increased much in size from what may be considered the ancestral 

 type, the organ is not found to have increased proportionally, and 

 though still retaining the typical sensory character it is in a 

 measure degenerate. Then, again, in all forms apparentl}' there 

 are mucous glands in connection w ith the organ and which dis- 

 charge into it. In small forms, e.g., Mus, Petaurus, Minioi)terus, 

 ttc, the glands are few and mostly situated at the posterior end 

 of the organ; while in relatively lar-ger forms as Lepus, Trichosurus, 

 ttc, the glands are numerous and open into the organ along nearly 



