BIONOMICS. 301 



keep to the outside of the host (e.g. fish-lice), and suck 

 nourishment by their mouths ; the Rhizocephala (e.g. 

 Sacculind) send ramifying absorptive roots through the 

 body of the host. Sometimes the parasitism is temporary 

 (Argidus] ; 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 host, 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 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 sea-weeds 

 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. Fight- 

 ing is very common. 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 Schizo- 

 pods) ; 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, such as 

 wood-lice and land-crabs, are terrestrial, and some blind 

 forms occur in caves. 



