Chapter 7. 



Systematio guide to the genera and speoies of Rhizooephala, 



with anatomical diagnoses. 



The difficulty which besets this subject is very great, because the simple structure of 

 the adults does not offer any characteristic features by which the difFerent species of the 

 various genera can be sharply distinguished. The Nauplius and Cypris larvae, from which 

 something might be hoped, do not as a matter of fact help us out: thus the Nauplius and 

 Cypris of the so-called Sacculina neglecta, parasitic on Inachus scoipio, are, as far as I can make 

 out, identical in structure with those of Sacculina carcini. parasitic on Carcinus maenas, and if 

 we unite these species together as one, the rest of the so-called species of Sacculina must be 

 given up. The shape of the adult body has been commonly userl as a specific criterion, but 

 this is greatly innuenced by the shape of the host's body which varies with sex and age. 



Another obvious ' criterion is the fact that the Uhizocephala infest special hosts, and 

 the ; question arises whether any absolutely general rule can be laid down that each sjiecies 

 of parasite only infests a single species as host, and never by any chance infests an host of 

 a difFerent species. Of course by artificially applying this rule, the systematic definition of 

 the Uhizocephala would be immensely simplified though greatly elongated, since it would be 

 only necessary to invent a new name for each parasite found on a distinct species of host. 

 But it is easy to convince oneself that this method would be most highly artificial, because 

 it would involve giving several specific names to forms which in ali stages of their existence 

 are quite inseparable by any structural detail. To take an instance, the species of Peltogaster 

 which is fairly common at Naples, P. curvatus (Kossmann), is found occasionally on Eupagurus 

 ■prideauxii (Leach) and much more commonly on E. meticulosus (E-oux). I can find no means 

 of distinguishing the parasites on these two hosts, and since they occur together on the sanie 

 ground there can be little doubt that the specimens found occasionally on E. prideauxii are 

 really of the sanie species as those commonly found on E. meticulosus. It is curious that 

 the parasite should be more frequent on E. meticulosus because this hermit-crab is much rarer 



