CRUSTACEA. 



229 



of recapitulation offers a philosophical rather than a material 

 explanation of the facts. 



Habits and Habitats. — Most Crustaceans are carnivorous 

 and predacious ; others feed on dead creatures and organic 

 debris in the water ; a minority depend upon plants. 



Parasitism occurs in over 700 species, in various degrees, 

 and of course with varied results. Most of the parasites keep to 

 the outside of the host (1?.^., Fish-lice), and suck nourishment 

 by their mouths or by absorptive roots (Rhizocephala, e.g.., 

 Sacculind). Sometimes the parasitism is temporary (Ar- 

 giilus) ; 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 prolific. The hosts, e.g., crabs infested by Rhizo- 

 cephala, are sometimes materially affected, and even ren- 

 dered 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. The hermit-crab is con- 

 cealed 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 of 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, as to the frequent 

 deliberateness of masking, 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 intelligenceof crabs and some of the higher Crustaceans 

 is well developed. Maternal care is frequent. Fighting is 

 very common, but the loss of limbs is readily repaired. 



Deep-sea Crustaceans are very abundant, and often remark- 

 able " for their colossal size, their bizarre forms, and brilliant 

 red colourings ;" some are blind, others are brilliantly phos- 

 phorescent. Yet more abundant are the pelagic Crustaceans 

 (especially Entomostraca and Schizopods); they are often 

 transparent except the eyes, often brightly coloured or phos- 

 phorescent. Many Crustaceans live on the shore, and con- 



