BIONOMICS. 283 



doubt that the metamorphoses of these Crustaceans is to some extent 

 interpretable as a recapitulation of the racial history, for there were 

 unsegmented animals before segmented forms arose, and the Zona stage 

 is antecedent to the Mysis, &c., yet it does not follow that ancestral 

 Crustaceans were like Nauplii. On the contrary, the Nauplhis must be 

 regarded as a larval reversion to a type much simpler than the ancestral 

 Crustacean. Moreover, this idea of recapitulation offers a philosophical 

 rather than a material explanation of the facts. 



Bionomics. 



Most Crustaceans are carnivorous and predatory ; 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 {e.g., Fish lice), and suck 

 nourishment by their mouths ; the Rhizocephala {e.g., 

 Sacculina), send ramifying absorptive roots through the body 

 of the host. Sometimes the parasitism is temporary {Ar- 

 gulus) ; sometimes only the females are parasitic {e.g., in 

 Lerncea). The parasites tend to lose appendages, segmen- 

 tation, sense organs, &c., but the reproductive organs become 

 more fertile. 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 

 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. 



