44 Dr. F. Miiller on the Rhizocepbala, 



VII. — On the Rhizocephala, a new Group of Parasitic Crustacea, 

 By Dr. Fritz Muller, of Desterro*. 



[Plate II.] 



Rathke's ' Beitrage zur Fauna Norwegens ' close with the de- 

 scription of two animah, PeltogasterPaguri and P. Carcini, which, 

 even on reading the admirable memoir, appeared to me to be the 

 most remarkable of the whole collection, and have since claimed 

 one of the first places in the series of animals which I desired to 

 investigate for myself. For this investigation an opportunity 

 was recently afforded me by the discovery of two nearly allied 

 species ; its results were partly so surprising and out of the 

 circle of our ordinary ideas, that, in communicating them, it is 

 really a comfort to me to know of the existence of the two allied 

 forms on the European coasts, and to be able to refer other na- 

 turalists to these in order to test my statements. 



The head of these apparent worms, which is inserted into the 

 body of the host, emits roots like those of plants — hollow tubes 

 which, being much ramified, cling round its intestines; and 

 their brood holds a middle place between that of the Lernsese 

 and Cirripedes. They therefore constitute a new section of pa- 

 rasitic Crustacea, to which I give the name of Rhizocephala from 

 the above peculiarity. It is to be expected that in these Rhizo- 

 cephala a rich mine of new forms will be opened up, as each of 

 the two Crabs which I have hitherto been able to examine in 

 large numbers nourishes a species. Unfortunately I am desti- 

 tute of all means of determining these animals; but they will be 

 easily recognized by future visitors to our coast without any de- 

 tailed description. Almost under every stone they will find a 

 blackish-green, smooth-clawed, and uncommonly nimble Porcel- 

 lana, and scarcely less abundantly, a small Pagurus^ which seeks 

 shelter almost exclusively in the shells of a Cerithium. 



The parasite of the Porcellana may be called LerncEodiscus 

 Porcellance ; and that of the Hermit Crab, Sacculina purpurea. I 

 shall describe first the two sexually mature animals, and then 

 their larvae. 



LerncEodiscus Porcellana (figs. 1-3) occurs pretty commonly, 

 usually single, rarely two together, attached to one of the ante- 

 rior segments of the tail of its host, and often completely fills 

 the space between the tail and the sternum. It has the form of 

 a fleshy and pale-yellowish flesh-coloured disk, which may be 

 upwards of 10 millims. in breadth by a little less in length. In 

 front and behind, the disk is deeply sinuous, and on each side 

 divided into from five to seven lobes, the usually enlarged ex- 



* Translated by W. S. Dallas, F.L.S., from Wiegmann's Archiv, 1862. 



