GENERAL NOTES ON CRUSTACEANS. 279 



as a Protozooea, with a cephalothoracic shield and an unseg- 

 mented abdomen. Thus there are two grades between Nauplius 

 and ZoEea. 

 Three facts must be borne in mind in thinking over the life histories 

 of crayfish, lobster, crab, and Penceus : (1) There is a general tendency 

 to abbreviate development, and this is of more importance when meta- 

 morphosis is expensive and full of risks ; (2) there is no doubt that larvse 

 exhibit characters which are related to their own life rather than to that 

 of the adult ; (3) it is a general truth, that in its individual development 

 the organism recapitulates to some extent the evolution of the race, that 

 ontogeny tends to recapitulate phylogeny. But while there can be no 

 doubt that the metamorphosis 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 Zocea stage 

 is antecedent to the Mysis, etc., — yet it does not follow that ancestral 

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

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

 Crustacean. Moreover, the idea of recapitulation offers a philosophical 

 rather than a material explanation of the facts, and holds good only in 

 a very general way. 



Bionomics. — Most Crustaceans are carnivorous and pre' 

 datory; 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. 

 Sacculind) 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 

 Lerncza). 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 

 Rhizocephalaj 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. 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 



