98 ANATOMY OF THE WOOD RAT 



any muscle of their own, renders it probable that the dis- 

 charge of the liquid contained is not under voluntary con- 

 trol of the animal. The fact that they seem to be absent 

 in the male suggests the hypothesis that they may be a 

 secondary sexual character of the females, perhaps secreting 

 a scent to attract the males during the breeding season. 



Some discussion of variation of the alimentary tracts of 

 related mammals with diverse food habits has been offered 

 elsewhere (1925) by the present writer. It has now been 

 argued that Homodontomys feeds chiefly upon nuts and 

 seeds; the species of Neotoma investigated lives where 

 vegetation is relatively scanty and its menu of seeds must 

 often be supplemented with anything of an edible nature 

 available; while Teonoma is largely herbivorous. The 

 stomachs of the three subgenera reflect the effects of this 

 fare in their degree of muscularity. Homodontomys has 

 the least speciaUzed stomach, that of Neotoma is consider- 

 ably more muscular, though simpler in contour, while that of 

 Teonoma is complex in shape and highly muscular. Size 

 of stomach alone is of little significance, as digestion takes 

 place in that, the small intestine and caecum. 



The length of the small intestine in Teonoma is greatest 

 of the three subgenera, as might be expected, but the fact 

 that it is shortest in Neotoma is disconcerting. The in- 

 creased diameter of the caecum may offset this, or there 

 may be more efficient glandular action of the small intes- 

 tine. There is not really sufficient difference in the caeca 

 of the three subgenera for the drawing of significant con- 

 clusions. 



But little actual digestion takes place in the rectal tract 

 and for this reason moderate differences in food habits may 

 affect it to a relatively shght extent. Be that as it may, 

 the rectal tract of Neotoma is but slightly more than half 

 that in the other two subgenera. 



Thyroid gland. This, in Homodontomys and Neotoma, 



