PARASITISM. 75 



We may classify the dwelling habits of animals into 

 Physical Partnership and Organic Partnership, thus : — 



PH \ 'SIC A L. ORG A NIC, 



1. Similar Organisms. 

 Gregarious. Colonial. 



2. Dissimilar Organisms. 

 Equal — Commensalistic. Symbiotic. 



Unequal — Ectoparasitic. Endoparasitic. 



Endoparasitism. — All animals which adopt the endo- 

 parasitic habit acquire certain features in common by 

 adaptation to • their pecuhar surroundings. 



In following out these features we may divide endopara- 

 sites into two groups : — (i) Somatic and (2) Enteric. 



I. Somatic. — Somatic endoparasites live in the body of 

 their host, usually in the muscles or one of the organs, e.g.^ 

 the liver. They feed upon the actual substance of the host, 

 and are therefore provided with a definite mouth and 

 alimentary canal. They may, in addition, be often provided 

 with locomotor organs. Their systems, which are most 

 modified, are the sensory, integumentary (skeletal), and 

 reproductive. Living inside their host, all sense-organs are 

 to them superfluous as they are removed from contact with 

 the outside world. In a similar manner all protecting 

 integuments, or exoskeletons, are superfluous. Many crus- 

 tacean parasites, whose free-swimming allies have a hard 

 calcareous skeleton, have a soft colourless skin. A loss of 

 colour is also usual. Lastly, an endoparasite requires well- 

 developed reproductive organs. Both sexes are usually 

 represented in one individual, owing to difliculties of fertilisa- 

 tion, and enormous numbers of eggs are also required. The 

 number of individuals in one host must be strictly hmited, 

 or the host would perish and with it the parasites. Hence 

 the young are forced to seek fresh hosts, and the difliculties 

 and perils of the migration are such that a high fecundity 

 can alone counteract the danger of extinction. A common 

 device is the invasion of an intermediate host, which itself 

 forms an article of food to the original host. If the inter- 

 mediate host be not an article of the original host's diet. 



