On the Rhizocephalan Genus Thompsonia, etc. 27 



ancestor with many external sacs of normal Rhizocephalan type. A 

 likely scheme of evolution is the following : 



The typical Rhizocephalan with single external sac and single root system. 



v 



Rhizocephalan with single root system but several external sacs of 



normal structure. 



v v _ 



Peltog aster socialis. Thompsonia. 



Separate external sacs and root A single root system and many externa 

 systems. sacs of simple structure. 



In this remarkable life-history we find many phenomena which could 

 hardly have been expected in so high a phylum as the Arthropoda. 

 The analogy with the Fungi is strikingly shown in the mycelium-like 

 root-system, producing its singular asexual reproductive organs, which 

 are not dissimilar to the sporangia of a form like Mucor. A more con- 

 vincing parallel may be drawn with a lower animal phylum, the Ccelen- 

 terata, where typical Hydromedusan forms, like Obelia and Podocoryne, 

 produce medusa buds with a complex structure and germ cells differ- 

 entiated in situ; while there are other genera, like Eudendrium and 

 Aglaophenia, in which the medusoid individuals are entirely degen- 

 erate, with the simplest structure, and the germ cells are formed in 

 the ccenosarc and migrate thence into the medusoids. If Sacculina 

 and Peltogaster correspond roughly to the former class, Thompsonia 

 occupies a place similar to that of the latter. 



THOMPSONIA AND THYLACOPLETHUS. 



When Coutiere established his genus Thylacoplethus he remarked 

 that in form it approaches Thompsonia globosa Kossmann: 



"Mais il doit en etre separe generiquement, meme avant toute comparaison 

 de la structure interne; Thompsonia a ete trouvee fixee sur les pattes d'un 

 Crabe (Melia tesselata Latr. provenant des Philippines), c'est-a-dire sur un 

 Crustace tres eloigne des Alphe"es dans la systematique, et d'autre part Thyla- 

 coplethus offre le premier exemple d'un Rhizocephale aussi completement 

 gregaire." 



In spite of the very incomplete description of Kossmann's parasite, 

 Hafele is almost certainly correct in identifying the Japanese form 

 described by him as Thompsonia. There is only one serious discrep- 

 ancy to be explained. In the type T. globosa only two external sacs 

 were found upon the original host, while T. japonica is as " gregarious" 

 as Coutiere's Thylacoplethus. This is not a point, however, which 

 weighs with me at all, for I know how easily these sacs are detached. 

 There were probably a larger number present at the time of capture. 

 The hosts were in both cases brachyurous Decapods and in both cases 

 the parasites were attached to the thoracic legs. It seems, however, 



