458 Mr Potts, Thompsonia, a little known Crustacean Parasite. — 
of cytoplasm in the lacunar space of the peripheral roots has been 
noticed before. When a club-shaped swelling is formed at the 
end of a rootlet these cellules appear to migrate into its interior 
and form the ovary of the future external sac. An interesting 
parallel may be drawn between Thompsonia and those Hydro- 
medusae in which the gonophores are degenerate in structure 
and the reproductive cells are formed in the coenosare, migrating 
thence into the gonophores. 
A curious thing is noticed on further examination of the — 
peripheral roots. That is the occurrence, among these very small — 
cellules, of much larger bodies, which appear to be segmenting ~ 
eggs corresponding in size and structure with those found within | 
the external sacs. I have not satisfied myself that the stage of 
development of these bodies corresponds to that of the ovarian 
eges. But what can be their fate? Unless they migrate into the 
external sacs they must disintegrate and disappear before develop- 
ment proceeds much further. What are the conditions which 
stimulate the reproductive cells to development whatever their 
situation in the organism? It must however be noticed that 
they are far fewer than those developing within the external sacs. 
The new light which these observations shed upon Thompsonia 
show it to be, not a primitive Rhizocephalan as Coutiere and — 
Hafele maintain it to be, but on the contrary the most specialised _ 
member of the group. The simplicity of the structure of the — 
external sacs is a secondary condition. Sacculina and Peltogaster 
still show in their external sacs a certain morphological resemblance | 
to the non-parasitic Cirripede. But the external sacs of Thompsonia — 
are little more than ovaries placed externally to allow of the 
escape of the larvae. The adoption of parthenogenesis (I feel 
little doubt that this is the method of reproduction here, as in | 
Sylon and Mycetomorpha) has made it possible to dispense with 
testes and gonadial ducts. The fact that these external sacs can 
be produced with great economy of material allows the appearance | 
of successive crops. But assuming, as I thus do, that Thompsonia — 
with its highly specialised root system and peculiar method of re- 
production is descended from an ancestor, resembling Sacculina and | 
Peltogaster in many respects, can we in any way trace the course © 
of evolution? I think some clue is afforded by the consideration | 
of Peltogaster socialis.* This parasite of hermit crabs is rarely — 
if ever alone; often there are a hundred or more external sacs, 
But it differs from Thompsonia in two most important particulars. — 
Firstly, the external sac is well developed, being similar in almost 
all respects to that of other species of Peltogaster; and secondly, 
the root systems of adjacent external sacs, as well as the sacs: 
* Smith, Fauna u. Flora Golfe v. Neapel. Rhizocephala, 1906, p. 57. Compare 
also Peltogasterella socialis, Kriiger, if this form is really distinct. - ; 
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