ON THE RHIZOCEPHALAN GENUS THOMPSONIA 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- 
pyllobtus 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 lie 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 deferens opening into the manile 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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