360 PHYLUM ARTHROPODA 



Sacculina) send ramifying absorptive roots through the 

 body of the host. Sometimes the parasitism is temporary 

 (Argulus) ; sometimes only the females are parasitic (e.g. in 

 Lerncea). 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. 113). The hermit-crab 

 is concealed and protected by the sea-anemone ; the latter is 

 carried 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 brightly 

 coloured or phosphorescent. Many Crustaceans live 

 on the shore, and play a notable part in the struggle for 

 existence which is so keen in that densely crowded region. 

 The lower Crustaceans are abundantly represented in 

 fresh water, in pools, streams, and lakes. A few Crus- 

 taceans, such as wood-lice and land-crabs, are terrestrial, 

 and some blind forms occur in caves, 



