ON THE RHIZOCEPHALAN GENUS THOMPSON!! AND ITS 

 RELATION TO THE EVOLUTION OF THE GROUP. 



By F. A. POTTS, M. A. 



INTRODUCTION.* 



The Rhizocephala are a group of undoubted Cirripedes having 

 nevertheless a structure and life-history in which the departure from the 

 normal type is probably greater than in any other parasitic Crustacea. 

 They are found upon Decapod Crustacea and in the adult form have 

 lost all trace of segmentation and appendages. Each one consists of 

 an external sac communicating by a peduncle with an internal root 

 system which traverses the body of the host and absorbs food from the 

 blood. The absence of an alimentary canal and the development of 

 an absorptive root system are characters which have been independ- 

 ently acquired in other parasitic Crustacea, to wit, the Copepods Her- 

 pyllobius and Rhizorhina, which yet retain signs of segmentation and 

 vestigial appendages. Moreover, in some parasitic Isopods (Wanalia 

 and Cryptoniscus) and in the Cirripede Anelasma there is an incipient 

 root system, although the gut does not degenerate. The modification 

 of the reproductive phenomena is very considerable in the Rhizocephala, 

 for it has involved the supression of the male sex and the conversion of 

 the other into self-fertilising hermaphrodites or, in a few genera, 

 parthenogenetic females. 



The criterion of the Cirripede affinities of the Rhizocephala is to be 

 found in the Nauplius and Cypris stages, which occur in their early 

 larval history. Without the evidence of embryology it would be diffi- 

 cult to refer the adult even to the Crustacea. The external sac, in 

 Sacculina, consists of a mantle surrounding a visceral mass but separ- 

 ated from it by the mantle-cavity or brood pouch (which opens to the 

 exterior by a mantle opening) except along the surface of attachment 

 to the host, where there is a communicating mesentery. A nerve 

 ganglion and the small tubular testes he in the mesentery; the main 

 bulk of the visceral mass is occupied by the ovaries and there are present 

 on each side an oviduct and a vas defer ens opening into the mantle cavity. 

 Geoffrey Smith (9), by comparing the arrangement of these organs with 

 those of the typical Cirripede, has made a plausible attempt to homol- 

 ogise the external sac with the body of other Cirripedes. This brief 

 summary of the typical Rhizocephalan structure is designed to show 

 that, while structural reduction has proceeded far, it is by no means 



* Service as a Lieutenant in the English Army has prevented Mr. Potts from revising the 

 proofs of this paper. A. G. M. 



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