On the Rhizocephalan Genus Thompsonia, ete. 7 
PRELIMINARY DESCRIPTION OF MATERIAL. 
During the first few days of our stay at Murray Island at the North 
end of the Great Barrier Reef, I sought for the Rhizocephalan para- 
site of Alpheus amongst the species dwelling in burrows on the reef 
(such as A. edwardsi), but without any success. On September 30, 
Dr. H. L. Clark brought to me a few specimens of a dark-coloured 
Alpheid (Synalpheus brucei, sp. n.), which he had found sheltering 
amongst the arms of a large Comatulid. One of these was distinguished 
from its fellows by the possession of numberless little pink egg-like* 
sacs attached to the thoracic legs and contrasting strongly with the 
purple colour of the cuticle of the host. These sacs were found to 
contain Cypris larve, and so little doubt remained that I was dealing 
with the creature described by Coutiére under the name of Thyla- 
coplethus. 'The next few days proved that, on the south reef of the 
island of Maér, there lived under every loose stone and within the 
branches of the living coral a teeming population of crinoids, of which 
the majority belonged to the species Comanthus annulatum (Bell). 
Within the living basket formed by the arms of the crinoid, a pair of 
Alpheids, male and female, were generally to be found, and a small 
proportion of the whole population were infected with the parasite. 
So numerous were the crinoids and their commensal Alpheids that they 
could be depended upon to provide a sufficient quantity of material for 
the study of the parasite. Only toward the edge of the reef, however, 
were the crinoids numerous, and as the spring tides waned it became 
more difficult to locate the crinoid with the water glass, to plunge 
beneath the water which covered its retreat, to extricate, by means of 
the brutal hammer or the persuasive hand, the unwilling echinoderm 
from amongst branching corals, to which it clung with desperate 
energy, and finally to prevent the escape of the agile Alpheid when the 
crinoid was raised above water. It was such experiences, however, 
which gave additional zest to our life at Murray Island. 
I was eventually able to obtain nearly twenty Alpheids infected by 
Thompsonia in various stages of development. ‘The external sacs 
varied from tiny bodies barely 0.5 mm. long to those a little more than 
1 mm. in length and 0.6 mm. in breadth. In the latter stage the Cypris 
larvee, with their dark eyes, could be plainly made out through the 
mantle. They were nearly always situated on the limbs. Some 
Alpheids only showed them on the abdominal swimmerets, where they 
were to be distinguished from the eggs of the host by their inferior 
size and different colour, but in the majority of hosts they occurred 
both on the abdominal and thoracic limbs. On some of the ambula- 
tory limbs as many as a dozen were found, but in a late stage of develop- 
ment they were greatly crowded, as is shown in plate 1, figure 2. As 
*The eggs of the Alpheid are, however, green and are larger than the parasite. 
