General Morphology. 



2. General Morphology. 



The Rhizocephala, excluding certain doubtful forms which have been held to be linked 

 with them, are a compact group of parasites comprising a few genera, ali of which exhibit 

 a similar organization and habit of life. Ali the known genera except one, are parasites of 

 Decapod Crostacea; the Macrura, Anomura and Brachyura being ali liable to the attacks of 

 special kinds: the one exception is the curious genus Duplorbis, found by Dr. H. J. Hansen 

 in a collection from Greenland and described for the first time in this work, which infects 

 the Isopod, Calathura brachìata. 



The parasites are most frequently solitary, a single individuai infesting each host ; there 

 are however gregarious species such as Peltogaster socialis and Thylacoplethus which are never soli- 

 tary, and even in the typically solitary forms it very frequently occurs that two or more indi- 

 viduals of the parasite are found on one and the same host. 



In the adult state the Rhizocephala are partly ectoparasitic and partly endoparasitic, 

 the body consisting of two portions, an external sac containing the reproductive, muscular and 

 nervous organs, and an internai system of ramifying roots which absorb the nourishment from 

 the internai juices of the host. Ali the species, however, whose life-histories are known, pass 

 through a most elaborate metamorphosis before achieving this stage, being at first free-swimming 

 larvae and subsequently passing through a completely endoparasitic stage in the body of 

 the host. 



The external sac-like portion of the adult body frequently takes up the position on the 

 host which is normally occupied by the brood of eggs, and the colour and appearance of this 

 sac is frequently assimilated to the colour and appearance of the eggs, conspicuously in Pelto- 

 gaster (Piate 1 fig. 8). Moreover since the parasite in the majority of cases has the remark- 

 able property of calling forth the development of female characteristics in the male host, 

 this position of the parasite is almost always of advantage, because even when it is situated 

 on a male host it is frequently protected by the development of those structures of the host's 

 body which are intended to safeguard the eggs. These facts and the general appearance of 

 the various parasites on their hosts are illustrated on Piate 1. 



The Rhizocephala are known from almost ali seas and at ali latitudes from the Arctic 

 circle to the tropics, but the best known species come from the North Sea, from the English 

 and French Coasts and the Mediterranean. 



The morphology of our group might be treated in a purely descriptive method, but 

 the anatomy of Sacculina has already been elucidated by Delage (14) with detailed correctness, 

 and in the systematic part of this work full diagnoses of the genera are given ; we will therefore 

 here introduce a series of salient anatomical facts as evidence of phylogenetic relationship, 

 remembering that although every phylogenetic system is to a certain extent arbitrary, the mind 

 desires an ordered system and the imagination is aroused in endeavouring to reconstruct thè 



