THE METAMORPHOSES OF CRUSTACEA 85 



degenerate Cirripedes, although their structure in 

 the adult state gives little evidence of their affinities. 



A number of interesting problems in specula- 

 tive biology are suggested by the larval stages of 

 Crustacea. A full discussion of these problems 

 would involve matters too technical for these pages, 

 but some indication of the broader issues may be 

 attempted. 



The obvious question, Why do some Crustacea 

 pass through a complicated metamorphosis while 

 others do not ? is, like many obvious and simple 

 questions, one of the most difficult to answer. It 

 will be pointed out later, in dealing with the fresh- 

 water Crustacea, that one of the most general 

 characters of fresh-water animals as compared with 

 their marine allies is the absence of free-swimming 

 larval stages. This applies, for instance, to the case 

 of the Crayfishes and the marine Lobsters, and to 

 that of the River Crabs, as compared with those 

 which live in the sea. But it does not apply to all 

 fresh-water Crustacea, and, on the other hand, there 

 are many cases of direct development in marine 

 species. 



Some of the advantages gained by the possession 

 of free-swimming larval stages are obvious enough. 

 Many Crustacea which live on the sea-bottom, and 

 are not very powerful swimmers, have their progeny 

 scattered far and wide by winds and currents while 

 in the surface-living larval stages. In the extreme 



