76 ENDOPAKASITISM. 



a further migration has to be instituted, which is re- 

 inforced by a second reproduction, causing metagenesis {cf. 

 Disfomum), 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 

 usually characterised by : — 



(i) 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 aHmentary 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 : — 



(i) 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. 



TcBnia 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. 



