76 ENDOPARASITISM. 
a further migration has to be instituted, which is re- 
inforced bya second reproduction, causing metagenesis (cf 
Distomum), It is also usual for the eggs to be provided 
with yolk and a hard outside shell, to withstand the vicis- 
situdes of the outside world. Thus a somatic parasite is 
usualiy characterised by :— 
(1) Loss of sense-organs. 
(2) Loss of exoskeleton and pigment. 
(3) Hypertrophy of reproductive organs. 
2. ENTERIC.—An Enteric parasite may go considerably 
further in its adaptation. It is usually resident in the enteron 
or alimentary canal of its host, and is bathed on all sides by 
soluble and diffusible proteids prepared for the use of the 
host. Its alimentary organs are therefore superfluous and 
atrophy, absorption taking place through the skin. The 
intestine of higher animals has rhythmic (peristaltic) con- 
tractions which tend to drive egestive products to the 
exterior. Hence enteric parasites usually have organs of 
fixation, such as hooks or suckers, to attach them to the 
intestinal wall. All the characters of somatic parasites are 
also shared by enteric, hence the adaptations of enteric 
parasites read as follows :— 
(1) Loss of sense-organs. 
(2) Loss of skeleton and pigment. 
(3) Loss of alimentary organs. 
(4) Hypertrophy of reproductive organs. 
(5) Acquirement of fixative organs. 
Tenia and Gregarina are two good examples of highly 
adaptive enteric parasites. 
Protective Resemblance* and Mimicry.—One of 
the most interesting sections of bionomics is the study of 
these two phenomena. Protective resemblance comes under 
the first heading above (physical relations), for it covers 
the cases of resemblance between an animal and its sur- 
roundings. In mimicry an animal shows a resemblance to 
some other animal. In each case it is usually supposed that 
the animal obtains a benefit or immunity from ever-watchful 
foes by such resemblance. The simplest cases are those of 
protective coloration, in which an animal has the power to 
_ * Certain resemblances may be distinguished as aggressive rather than protec- 
tive as they are meant to attract the prey or to put it off its guard. 
