28 Papers from the Department of Marine Biology. 
that Thompsonia is not like the other genera of Rhizocephala, the 
occurrence of which is confined in each case within a very narrow 
division of the Decapods, but enjoys a very wide diversity of hospi- 
tality; for Kriiger reports that not only does 7’. japonica parasitise 
Brachyura, but is even found upon a hermit crab (Pagurus striatus). 
The two points, then, upon which Coutiére lays such great stress are 
shown to be without importance by later work. 
Why, then, did Hafele not recognise the identity of Thompsonia and 
Thylacoplethus? His negative decision rests upon points of difference 
in structure, but he makes some reservation owing to the unsatisfactory 
nature of Coutiére’s unillustrated account. I will quote his concluding 
sentence: 
“‘Gerade aber der Umstand, dass Coutiére von einem ‘Manteau 4 double 
paroi chitineux dans lequel est suspendue une masse viscérale, et qui porte 
une ouverture cloacale’ spricht, lisst mich mit ziemlichen Sicherheit vermuten, 
dass in Thylacoplethus eine ganz andere Gattung vorliegt.”’ 
I am at a loss to understand why Hafele did not find an inner layer 
of chitin (the internal boundary of the mantle), but I think it will be 
found to exist in 7’. globosa when better preserved material is examined. 
With regard to the existence or non-existence of a cloacal aperture, 
this apparent difference is due to the fact that the organ appears only ata 
very late stage in development. But in any case I hardly think that, 
in view of the obvious agreement in structure and development, there 
could be any reason for separating generically Thompsonia (as described 
by Hafele) and the forms parasitic on Alpheids (as described by 
Coutiére and myself). My own observations fail to reveal any differ- 
ence between the parasites of Brachyura and Alpheids and an exam- 
ination of the figures given by Hafele for Thompsonia globosa (for stages 
which the parasite of Thalamita does not show) and comparison with 
sections of corresponding stages in the Alpheid parasites show an 
essential similarity. 
There is one point to which I refer further in the next section in 
Coutiére’s description of Thylacoplethus, but may be briefly mentioned 
here. It is the occurrence of parasites on the sternum and not on the 
appendages of the host. But this difference can hardly be conceived 
to be of generic value. 
AMENDED DIAGNOSIS OF GENUS. 
Thompsonia Kossmann (= Thylacoplethus Coutiére). 
A colonial Rhizocephalan infecting various Decapod Crustacea. Root 
system diffuse, widely distributed in body of host, sending branches into 
thoracic and abdominal appendages, which give off numerous sacs containing 
reproductive cells and becoming external at a moult of the host. These 
external sacs consist of a mantle and visceral mass without an intervening 
mantle cavity: mantle thin and devoid of muscle fibres. Visceral mass con- 
tains ovary only without generative ducts, testis, or nerve ganglion. Develop- 
