FPARASITISM. 75 
We may classify the dwelling habits of animals into 
Physical Partnership and Organic Partnership, thus :— 
PHYSICAL. ORGANIC, 
1, SIMILAR ORGANISMS, 
Gregarious. Colonial. 
2. DISSIMILAR ORGANISMS. 
£qual—Commenaalistic. Symbiotic. 
Onegual—Ectoparasitic. Endoparasitic. 
Endoparasitism.—All animals which adopt the endo- 
parasitic habit acquire certain features in common by 
adaptation to their peculiar surroundings, 
In following out these features we may divide endopara- 
sites into two groups :—(1) Somatic and (2) Enteric. 
1. SoMatic.—Somatic endoparasites live in the body of 
‘their host, usually in the muscles or one of the organs, ¢.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 difficulties of fertilisa- 
tion, and enormous numbers of eggs are also required. The 
number of individuals in one host must be strictly limited, 
or the host would perish and with it the parasites. Hence 
the young are forced to seek fresh hosts, and the difficulties 
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, 
