296 ANIMAL PHYSIOLOGY. 
tortuous or remain somewhat straight. The alimentary tract 
of a mammal passes through stages of development which cor- 
respond with the permanent form of other groups of verte- 
brates, according to a general law of evolution. Inasmuch as 
the embryonic gut is formed of mesoblast and hypoblast, it is 
easy to understand why the developed tract should so invaria- 
bly consist of glandular structures and muscular tissue dis- 
posed in a certain regular arrangement. The fact that all the 
Siding Diy 2”. Bs d oo Weis 
. i We | I : An an i is : 
au 
Fic. 254.—Portion of a jelly-fish, the Medusa Aurelia, showing gastro-vascular canals radi- 
ating from central stomach and terminating in a circular marginal canal (after Romanes). 
All these aré shaded very dark ; the light spaces indicate artificial sections. Inasmuch as 
these canals as well as the stomach must contain some sea-water, and since their contents 
represent the whole of the nutritive fluid (answering to the blood. lymph, and chyle of 
higher forms), we have both anatomically and physiologically a very crude or undifferen- 
tiated condition in such animals, and one of great interest from an evolutionary point of 
view. 
organs that pour digestive juices into the alimentary tract are 
outgrowths from it serves to explain why there should remain 
a physiological connection with an anatomical isolation. The 
general resemblance of the epithelium throughout, even in 
parts widely separated, also becomes clear, as well as many 
other points we can not now refer to in detail, to one who 
realizes the significance of the laws of descent (evolution). 
Comparative-—Amosba ingests and digests apparently by 
every part of its body; though exact studies have shown that 
it neither accepts nor. retains without considerable power of 
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