DIGESTION OF FOOD. 
299 
Before passing on to higher groups, it will be well to bear 
in mind that the digestive organs are to be regarded as the out- 
come both of he- 
redity and adap- 
tation to circum- 
stances. We find 
parts of the in- 
testine, e. g., re- 
tained in some 
animals in whose 
economy they 
seem to serve 
little if any good 
purpose, as the 
vermiform ap- 
pendix of man. 
Adaptation has 
been illustrated 
in the lifetime 
of a single indi- 
vidual in a re- 
markable man- 
ner; thus, a sea- 
gull, by being fed 
on grain, has had 
its stomach, nat- 
urally thin and 
soft-walled, con- 
verted into a 
muscular giz- 
zard. 
Since diges- 
tion is a process 
in which the 
mechanical and 
chemical are 
both involved, 
and the food of 
animals differs 
| 
\ 
14 
B\ \VD | 
OD ROOVyy SFT 
Lon: 
Pita teceay on 
ae asda 
sggaves 
G20 g.-6 
paaaHae 
BROS 
SBELAPSa 
SBEOSE » 
oo? a 
gH Seg 
980 gf 
E58 We 
Se a5%3 
=. 88 «i 
ESE Be 
ie O S.4 
Rah 54 
EBS, SE 
BEESON 
Bae ° 
&, - 
SUR 
Evy) 
part o 
'P, nephridium: OD 
ortion of nerve-collar cut across; NC, ventral nerve-chain ; N. 
; VS, posterior appendages of the median vesicul: 
itudinal vertical section through anterior portion of animal (after 
ity; C, crop; CU, cuticle: D, thickened cuticle of 
cecal cavi 
gitudinal muscles of body-wall; 
8) 
a retractile muscles of anterior 
on; 
4 
im; RD, receptaculum ovorum: SF, seminal funnel 
ML 
; VD, vas deferens 
; IN, intestine ; 
gizzard ; 
, prostorniu: 
typhlosole 
S, 
harynx ; P. 
a 
(common earthworm). 
1-24, first twenty-four segments, 
BC, bu 
Pp 
ciferous gland: the passage runs forward as indicated by the arrow 
efferens ; T, anterior testis ; 
, muscular wall of 
MG. 
pidermis ; G, gizzard 
Marshall and Hurst). 
canal ; EP, e] 
of body-wall ; 
nalis ; N, dorsal p 
OG, aperture of cal 
retractor muscles of 
Fie. 257.—Lumbricus terrestris 
so widely, great variety in the alimentary tract, both ana- 
tomical and physiological, must be expected. Vegetable food 
must usually be eaten in much larger bulk to furnish the 
needed elements; hence the great length of intestine habitually 
