4 THE VOYAGE OF H.M.S. CHALLENGER. 



for existence as tlie adults, they have undergone countless secondary modifications which 

 have no reference to the life of the adult, and are therefore unrepresented in the adult 

 organism ; and a comparison of the various larvae which are here figured and described 

 will show that they difi"er among themselves more than the adults, thus reversing the 

 general rule that larvse are less specialised and exhibit clearer evidence of genetic relation- 

 ship than mature animals. The problem which they present is very similar to, but more 

 diflficult than, that presented by the Hydro-medusae, for young Medusae can be reared 

 from the hydroids in aquaria without difiiculty, and it is also easy to rear young hydroids 

 from the eggs of Medusae, but the life-history of the Stomatopoda must be traced from 

 the internal and indirect evidence furnished by comparison. 



The Stomatopod larvae present difi'erences among themselves, and they may be 

 arranged in genera and species, but unfortunately their generic characteristics are quite 

 difi"erent from those upon which the adult genera are based, and this is true in a stiU 

 greater degree of their specific characteristics. As the larvse undergo great changes during 

 their growth, different stages have been described as distinct species or even genera, and 

 it is not easy to select from the rich gatherings which are brought home by collectors, the 

 successive stages in the history of a single species. Like the adults, they are widely dis- 

 tributed, and a gap in a series from the North Atlantic may be filled by a specimen from 

 the coast of Australia or the Sandwich Islands, and the collection from a single locality 

 may contain the larvae of several widely separated species of adults in all stages of 

 growth. 



The attempt to unravel the tangled thread of the larval history of the Stomatopoda 

 is therefore attended with very exceptional difiiculties, and the earlier writers were 

 content to rest after the bestowal of generic and specific names upon the larvae, and the 

 first writer to approach the subject in a scientific spirit was Claus, whose classical mono- 

 graph not only abounds in fundamental generalisations of the greatest interest and value, 

 but also contains nearly all that we know regarding the relationship between the larvae 

 and their adults ; but the Challenger collections, especially the rich collections of Alima 

 larvae, a group in which Claus's collections were very deficient, furnish the material for 

 revision of the subject, and enable us to determine, with much greater certainty than 

 before, the larval type which pertains to nearly every one of the genera of adult 

 Stomatopoda, and also to give a pretty complete picture of the developmental history of 

 each larval type. As the specific difi'erences between the adults are very slight, the 

 specific identity of each larva can be determined only by rearing the adult from the 

 larvae, but this fact renders it the more important that the series collected by the 

 Challenger should be figured and described, as later investigators will thus be enabled to 

 complete the history by keeping the final stage in each series alive in an aquarium until 

 it assumes the characteristic of the adult. This can be done without difl&culty, as the older 

 larvae are hardy, whUe the fact that the 3rounger larvae will not live in captivity 



