8 Papers from the Department of Marine Biology. 
recorded by Hifele,they are clustered thickly on the proximal part of 
the limb, while the last two or three joints are entirely free. Occasion- 
ally external sacs are found on the chelipeds and even the third pair of 
maxillipeds, though I have never seen them, as Hafele notes, on the 
eye-stalks of the host. Posteriorly they extend to the uropods and to 
both surfaces of the telson when the infection is heavy. Rarely, too, 
they spring from the pleura of the abdominal segments, but never from 
the terga and sterna. 
On the last day of work at Murray Island (October 25) I obtained a 
swimming crab, Thalamita prymna, which was parasitised by another 
species of Thompsonia (pl. 1, fig. 3). The thoracic legs, including both 
the chelz (text-figure 1), were thickly beset with elongated lemon-yellow 
Fie. 1. 
Chela of Thalamita prymna_ para- 
sitised by Thompsonia. X2. The 
number and thick distribution of 
the external sacs along the margins 
of the appendage are shown about 
as in life. 
sacs containing Cypris larve, much larger than those on Synalpheus. 
The sacs were nearly 3 mm. in length, counting the peduncle. It 
was impossible, as we were on the eve of departure, to obtain further 
material of this form, but the capture gave me an opportunity of 
examining the genus on its typical host, a Brachyuran. Since the swim- 
ming crab is much larger than the Alpheid, the number of external sacs 
is much greater on the former; a single ambulatory limb may have 
nearly a hundred external sacs upon it. Altogether there were cer- 
tainly more than 500 on my specimen of T’halamita, while the number 
on Synalpheus never reaches 200. 
Dr. W. T. Calman, of the Natural History Department of the 
British Museum, has been kind enough to entrust to me a specimen of 
Actea ruppellii (pl. 1, fig. 4) from South Africa, which also bears a large 
number of sacs of a species of Thompsonia. This species probably 
differs from that found on Thalamita, the sacs being much smaller and 
pear-shaped. 
As far as I am able to judge, there is no essential difference between 
the forms of Thompsonia which infest Macrurous and Brachyurous 
