6 Papers from the Department of Marine Biology. 



The most interesting point in Coutiere's papers is his daring sup- 

 position as to the significance of Thylacoplethus in the origin and evolu- 

 tion of the Rhizocephala. He remarks that the adult parasites exist 

 on the pleopods in almost the very situation in which the attached 

 larvae of Sacculina* are found, and also that the root systems of adja- 

 cent parasites, if not entirely independent, are at any rate largely 

 localised. These observations show that here at least infection of the 

 host takes place by direct metamorphosis of the larva into the adult 

 at the position of fixation, without the intervention of such a stage of 

 internal parasitism as characterises the life-history of Sacculina. This 

 conclusion points to Thylacoplethus as a primitive or even ancestral 

 form and explains the "gregarious habit" as due to the simultaneous 

 fixation of large numbers of larvae. 



The latest researches on Thompsonia have been made by Hafele (4) 

 and Kriiger (5) on material brought back by Professor Doflein of 

 Munich from Japanese waters. The host in the larger number of 

 cases was a species of Pilumnus (a Xanthid crab). Hafele was not 

 able, owing to the absence of illustrations in Coutiere's rather meagre 

 descriptions, to decide whether his form was identical with Thylaco- 

 plethus^ or not. But the possession by the latter of a distinct mantle 

 and a cloacal opening led Hafele to suppose that the two forms might 

 be safely placed in different genera. 



In this paper the first attempt is made to give adequate figures of 

 the parasite. Series of sections were cut to trace the course of the 

 root system, but unfortunately a curious error of identification of the 

 root tissue is made. This is corrected by Kriiger from examination of 

 material in the same collection. But both authors agree with Coutiere 

 in supposing that each of the external sacs is an individual formed by 

 metamorphosis from a Cypris larva. Kriiger alone seems to contem- 

 plate the existence of an internal stage in the life-history as a possibility. 



In 1913 Dr. A. G. Mayer, Director of the Department of Marine 

 Biology of the Carnegie Institution of Washington, kindly invited me 

 to accompany the expedition of his department to Torres Straits. In 

 arranging my programme for this purpose, it occurred to me that 

 Coutiere's observations were partly made on specimens from the very 

 field of work to which we were journeying. I kept in view, then, the 

 interest which an examination of fresh and specially fixed material of 

 this form could not fail to possess, and was fortunate enough to procure 

 specimens which throw a great deal of light on this obscure form. 



*In reality the larva? of Sacculina do not fix more readily on the appendages than elsewhere. 



t" La faible extension des racines, leur localisation dans les bourrelets saillant, dus u 1'irritation 

 causee par les parasites le grand nombre de ceux-ci, leur fixation dans une cupule deprimee de 

 dehors en dedans, la presence de parasites adultes tres voisins sur les pleopodes d'A. malleodigitatus, 

 ou de Thompsonia sur les pattes d'un Crabe la ou s'effectuerait surtout 1'inoculation des larves 

 de Sacculine au stade Kentrogone; tous ces faits montrent que, chez Thylacoplethus au moins, 

 1'inf estation de 1'hote doit se f aire par fixation directe des larves a leur place definitive, sans 1'inocula- 

 tion ni le stade de parasite interne qui paraissent caractriser 1'evolution de Sacculina carcini 

 Thompson." 



