f)0 



FIRST LESSONS IN ZOOLOGY 



tubes, etc. 



Worms (figs. 55 and 56), crabs, and snails 

 (^ have a definite alimentary canal 



\ with certain parts set apart for 



I special processes. In the verte- 



brates the digestive apparatus 

 varies from a relatively simple 

 straight tube to the very long 

 and complex alimentary canal of 

 the cow (fig. 57). All these forms 

 depend much on the nature of the 

 food of the individual animal, and 

 the processes necessary to turn 

 it into body material. 



To illustrate what complexity 

 of the digestive sj'stem may be 

 reached let us consider somewhat 

 in detail the structure of the ali- 

 mentar\' canal of a small mammal 

 -a rabbit, for example. First is 



Fig. 54. — Diagnim i.f sec- 

 tion tliroiigli a simple 

 Fir:. 53._Snikirif; proboscis of a spliinx P"lyp, O/v/ia, sliowing 



iiL.tJi; ill small figiin- tli,' proboscis is lUgestive cavity; m, 



sliowri roil..] upon (ho nn.lor si.lo ot the- nioutli -opening ; ,;/.<-, ali- 



h.a.l, llio normal |iosilion wji.-n not in use. incnt.irv cavity. (Alter 



(( )no-lialf nalnral size; Irom specimen.) Parker and llaswell.) 



tlic mouth with its parts; the walls of the mouth are 

 furnished with minute tube-like extensions or glands. 



