1 aq Systematic guide to the genera aud species of Rhizocephala, with anatomical diagnoses. 



Sacculina is distributed all over the world, and the systematise in Norway who finds a 

 small specimen on a northern Spider-crab may smile when he is asked to believe that it is 

 the same species as a large form on the Mediterranean Eriphia or on a tropical crab, but he 

 must remember that the distribution of Sacculina from the Arctic circle to the Tropics is per- 

 fectly continuous into various depths, and that intermediate forms can be found between all 

 shapes and sizes. I do not however wish to dogmatise and to hold rigidly to the theory 

 that there is only one species of Sacculina, but I do wish to make a practical protest against 

 the method that has hitherto been followed, namely that of picking up a specimen of Saccu- 

 lina on a new host and giving it a new name without attempting to point out characters of 

 specific value that separate it from other forms. This otiose practise is not only idle but 

 positivelv mischievous, because it has led in a number of cases to the neglect of real ana- 

 tomical characters and to the confusion of specimens of Sacculina with those of Lernaeodiscus 

 and other genera, which resemble Sacculina only in outward appearance. 



That there may exist distinct races of Sacculina adapted to live as parasites only on 

 particular species of Brachyura is highly probable, but until a scheme of classification based 

 on anatomical distinctions is proposed, it appears to me idle to give separate names to 

 these races. 



In dealing with the genus Sacculina I have reproduced for purposes of reference the 

 names that have been given to the parasites of various crabs. 



The genera Thompsonia, Apeltes and Thylacoplethus I have been unable to examine 

 myself, and I remain in doubt as to many of their characters. 



Order: Rhizocephala (Muller 6). 

 (= Suctoria [Lilueborg 4] = Kentrogonidae [Delage 16].) 



Diagnosis. Degenerate Cirripedes, parasitic on Crustacea, passing through a Nauplius 

 and Cypris stage: fixation on the host occurring at the end of Cypris stage. The Cypris 

 assumes the Kentrogon stage and infests the hosts body with a little mass of undifferentiated 

 embryonic cells, which now go through a period of endoparasitic life, multiplying to form a 

 central tumour and a root system. The part of the adult body which becomes external is 

 differentiated from the central tumour, while the roots remain inside the host and supply the 

 parasite with nourishment. 



The evaginated, external part of the body consists of a mantle, surrounding a visceral mass, 

 the two being joined along the dorsal surface by a mesentery. The nervous system is reduced 

 to a single ganglion, Avhich lies dorsally and anteriorly. The body is fixed to the host by a 

 peduncle and ring of attachment, homologous to the stalk of the C'irripedia. Alimentary canal 

 degenerates immediately after formation by invagination in the embryo. Adult, hermaphrodite 

 or female Syhn . complemental males ilenvnerate or perhaps functional [Duplorbis . 



