Systematic guide to the genera and species of Rhizocephala, with anatomical diagnoses. 107 



than E. prideauxii. It is to be observed, however, that the shells in which E. meticulosus lives 

 are very often destitute of an anenome, and never have such large anenomes as E. prideauxii '; 

 and it is probable that this would favour the fixation of the Cypris larva on E. meticulosus. 

 But perhaps there is a more recondite reason. 



As a further instance of one species of Peltogaster infesting more than one species of 

 host, P. sulcatus (Lillj.) may be mentioned which is found at Naples indifferently on E. pri- 

 deauxii and E. meticulosus; nor can I distinguish the Neapolitan forms from those found on 

 various Norwegian Pagurids. 



From these considerations and from many more that could be given, the rigid appli- 

 cation of the rule that a special species of parasite is always attached to a special sjiecies of 

 host is impossible. 



The method I have followed in dealing with the numerous so-called species of Sacculina, 

 will, I expect, be adversely criticised by many system atists, but after several attempts to discern 

 constant specific differences between several forms that have been given distinct names, I have 

 come to the conclusion that all the forms of Sacculina are only races of one true species, 

 which may be called Sacculina carcini 'Thompson). The present state of the systematics of 

 this genus is most unsatisfactory ; the majority of so-called species are simply characterized by 

 the host on which they were found, or at most the external shape of the body is described, 

 an utterly worthless character which varies with the age of the parasite and the nature, 

 age, and sex of the host. The only attempt at a diagnosis of certain forms is made by 

 Kossmann (11) who combines an use of a number of characters, such as the rugosity of the 

 mantle, and the shape of the body, which I believe to be quite worthless, with a few ana- 

 tomical differences scattered here and there among the species, which, even when they are 

 not purely imaginary, always betray a lack of systematic tact. Thus he separates Sacculina 

 hians from other species, largely on the ground that it possesses a tube in the visceral mass 

 running from the ring of attachment and opening in the mantle-cavity, which he takes for 

 the representative of the gut; a character, which if it were not imaginary, would suffice to 

 remove Sacculina hians, not only from the genus Sacculina, but from the suborder Rhizocephala. 

 From Kossmank's figure and description I suppose his Sacculina hians to be a form of Hetero- 

 saccus, and the tube which he took for the gut is merely a lacunar space underneath the 

 ring of attachment, which needless to say does not open into the mantle-cavity and has nothing 

 at all to do with a gut. Another species, Sacculina papilio, on his own showing, is manifestly 

 a species of Lemaeodiscus: and although Kossmann has an inkling of this in supposing it to 

 be an intermediate form between Sacculina and Lemaeodiscus, an examination of its anatomy 

 would have shown the impossibility of retaining it in the genus Sacculina. The exposition of 

 these grave errors is sufficient to throw doubt on the whole of Kossmann's systematic work. 



I have examined the microscopic characters of the mantle, both externally and inter- 

 nally, and of the retinacula in the Neapolitan forms of Sacculina, without being able to find 

 constant differences of specific value. 



14* 



