On the Rhizocephalan Genus Thompsonia, etc. 5 
forms of this genus which infest Alpheids, under the name of Thyla- 
coplethus, has reverted to the theory of Giard, supposing that each of 
the external sacs represents a Cypris larva which has fixed and meta- 
morphosed in that identical position. The simple structure of the sac 
is another primitive feature, and we are led to believe that the more 
complex structure of Sacculina is due to progressive evolution from a 
form like Thompsonia. This view has also been taken by the few 
workers who have studied material of this Indo-Pacific form. The 
conclusions which are arrived at in this paper combat decisively the 
claim that the genus represents in any way the ancestor of the group 
Rhizocephala. As is so often the case, a form which seems at first 
sight to fill a gap between a group and its relations proves on closer 
examination but to illustrate an extraordinary capacity for specialised 
development in the group to which it belongs. But though the prob- 
lem of Thompsonia thus changes with widening evidence, it is yet a 
form of great importance to the student of comparative anatomy and 
general biology. 
PREVIOUS WORK ON THOMPSONIA. 
In 1874 Kossmann (6) published a description of the Rhizocephalous 
Crustacea collected by Semper in the East Indies. In this class he 
placed a parasite, two examples of which were found on the walking 
legs of Melia tessellata. He named this animal Thompsonia globosa, the 
generic name being given in honour of that distinguished Irish natur- 
alist who first recognised the Cirripede affinities of the Rhizocephala. 
The new genus was characterized by its globular shape and small size, 
long stalk with peculiarly thickened chitinous ring, absence of a mantle 
opening, reduction of the visceral mass, and retention of the larve 
until the Cypris stage was reached. Only two examples of the parasite 
occurred on this host, so the most striking feature of the genus, the 
presence of large numbers of external sacs of equal development, is 
not here recorded. 
In 1902 Coutiére (1) described a new type of Rhizocephalan parasitic 
on various Alpheids from East Indian and Australian waters and re- 
markable for the fact that each host carried under its abdomen, attached 
to the appendages, a great number of parasites, which resembled the 
eggs somewhat in volume and aspect. This parasite, for which he 
proposed the name of Thylacoplethus (several species of the genus 
were described) approaches T’hompsonia in its form, but must, he says, 
be separated generically, first and foremost because its host, the shrimp 
- Alpheus, is so different from the host of Thompsonia, which is the crab 
Melia. Besides, Thylacoplethus is distinct as the first example of a 
truly gregarious Rhizocephalan. I shall deal later with my own 
reasons for setting aside these conclusions of Coutiére and treating 
the name Thylacoplethus as a synonym of Thompsonia. 
