316 PHYLUM ARTHROPODA. 
animalcules and débris, they are themselves the food of 
larger animals such as fishes. ; 
Parasitism occurs in ovetZe0 species, in various degrees, 
and, of course, with varied results. Most of the parasites 
keep to the outside of the host (e.g. fish-lice), and suck 
nourishment by their mouths; the Rhizocephala (e.g. 
eater, weend_zamifving—absorptive roots through the 
ody of the host. Sometimes the parasitism 1s temporary 
( Areulus); sometimes only the females are parasitic (e.g. in 
Lernea). The parasites tend to lose appendages, segmen- 
tation, sense organs, etc., but the reproductive organs 
become more fertile. The hosts, e.g. crabs, infested by 
Rhizocephala, are sometimes materially affected, and even 
rendered incapable of reproducing. 
Some Crustaceans live not as parasites, but as commensals 
with other animals, doing them no harm, though sharing their 
food. Thus there is a constant partnership between some 
hermit-crabs and sea-anemones (Fig. 16). The hermit-crab is 
concealed and protector vy ane Sea anemons—thelaiter is 
camied about by the Crustacean, and gets fragments_of 
food. 
Masking is also common, especially among crabs. Some 
will cut the tunic off a sea-squirt and throw it over their own 
shoulders. Many attain a mask more passively, for they are 
covered with hydroids and sponges, which settle on the 
shell. There is no doubt, however, that some actively 
mask themselves, for besides those known to use the 
‘Tunicate cloak, others have been seen planting seaweeds 
on their backs. The protective advantage of masking both 
in offence and defence is very obvious. . 
The intelligence of crabs and some of the higher Crus- 
taceans is well developed. Maternal care is frequent. 
Fighting is very common. Many will “voluntarily” part 
with a leg to save themselves from their enemies. The 
loss of limbs is readily repaired. 
Deep-sea Crustaceans are very abundant, and often 
remarkable “for their colossal size, their bizarre forms, 
and brilliant red colouring”; in many cases, they are 
brilliantly phosphorescent. Yet more abundant are the 
pelagic Crustaceans (especially Entomostraca and Mysids) ; 
they are often transparent except the eyes, often 
