16 THE MEDUSAE. 



different localities, yet a study of the distribution and development of the 

 two genera shows that this parallel is only apparent. Pelagia is a truly 

 oceanic genus, in which the so-called species are probably not geographically 

 constant. Charybdea, on the other hand, almost certainly passes through a 

 scyphistoma stage (Haacke, '87), and is usually recorded from harbors, inlets, 

 or, at the most, from but a few miles off-shore. Furthermore, its races seem 

 to be constant and are often isolated from one another. It is very doubtful 

 whether such slight and geographically inconstant variations as those of 

 Pelao-ia and 'many such oceanic Hydromedusae as Ehopalonema, deserve 

 any recognition in nomenclature. But in the case of such a geographically 

 restricted and local genus as Charybdea equally small variations, when not 

 representing mere developmental differences, are of much greater systematic 

 importance, and, if they prove to be constant for different localities, may 

 well be regarded as the basis for specific distinctions. Differences due to 

 different stages in growth must however be carefully guarded against; it 

 was such a one, simple or branched condition of the velar canals, on which 

 Haeckel ('80) founded the subgenera Charybdella and Charybdusa. 



The characters of greatest specific importance in the genus are size and 

 general form of adults, structure of the phacelli, number of the velar canals, 

 and their condition, whether branched or simple, if, indeed, simple canals 

 should prove to be characteristic of the adult of any species of the genus, 

 which at present seems doubtful. 



Owing to the facts that most of the older species were founded on such 

 unstable characters as the relative proportions of different parts of the bell, 

 that many of the specific descriptions are altogether inadequate, and that 

 practically nothing is known of the younger stages in growth of most of the 

 species, it would be hopeless to attempt any revision of the genus without a 

 study of extensive series of specimens from the type localities of the various 

 forms. 



I believe that it is very important that descriptions of specimens of such 

 genera as Charybdea should be as detailed and specifically diagnostic as 

 possible, whether new or old species be in question. Neglect of this pre- 

 caution, particularly in the case of Pacific Medusae, and the practice which 

 some writers follow of giving no reasons for their specific identifications, 

 even when these form the bases for important generalizations, have made 

 the study of the geographic distribution of the Medusae much more difficult 

 than it need have been. 



