RODENTIA 



447 



large 



some 

 Beaver, 





shaped sac with 

 other species, as in 

 glandular masses are 



stomach is much dilated, forming a 

 thickened glandular walls; and in 

 Lophiomys imhausi and in the 

 attached to and open into the 

 cardiac or pyloric pouches. The 

 alimentary canal (Fig. 19G) of 

 all Rodents, with the exception 

 of the Dormice (Myoxida ), has 

 a caecum, which is often of great 

 length and sacculated, as in the 

 Hares. Water- Voles, and Porcu- 

 pines. In some instances, as in 

 the Hamster and Water-Vole, 

 the long colon is spirally twisted 

 upon itself near its commence- 

 ment. The liver is typically 

 divided in all, but the lobes are 

 variously subdivided in the | 

 different species (in Capromys 

 they are divided into minute 

 lobules) ; and the gall-bladder, 

 though present in most, is absent 

 in a few. In most species the 

 penis (which is generally pro- 

 vided with a bone) can be more 

 or less completely retracted 

 within the fold of integument surrounding the anus, where it lies 

 curved backwards upon itself under cover of the integument. It 

 may, however, be carried forward some distance in front of 

 the anal orifice, from which in the breeding season, as in the 

 Voles and Marmots, the prominent testicular mass separates it. 

 The testes in the rutting season form projections in the groins, 

 but (except in the Duplicidentata) do not completely leave the 

 cavity of the abdomen. Prostatic glands and, except in the 

 Duplicidentata, vesiculse seminales are present in all. The uterus 

 may be double, each division opening by a separate aperture into 

 a common vagina, as in Leporidce, Sciuridce, and Hydroclmrus, or 

 completely two-homed, as in most species. The mammas vary in 

 number and position from the single abdominal pair of the Guinea- 

 Pig to the ten thoracico- abdominal pairs found in some of the 

 Mats. In the Ododoiitidu? the mammas are placed high up on the 

 sides of the body. 



The peculiar odour evolved by many Eodents is due to the 

 secretions of special glands, which may open either into the 

 prepuce, as in Mus, Arvicola, Cricetus, etc., or into the rectum, as in 

 Ardomys and Aidnrudus, or into the passage common to both, as in 



Fig. 196.— Alimentary canal of Rat (Mm decu- 

 maims), the greater part of the small intestine 

 being omitted, o, CEsophagus ; d, duodenum ; 

 i, ileum ; evi, csecum ; c, colon. 



