Mr Potts, Thompsonia, a little known Crustacean Parasite. 457 
layer, which becomes the mantle, is thicker, and its external 
cuticle is more developed, while the distinct central part, crowded 
with small nuclei, represents the visceral mass. A comparison of 
the cast skin with the appendage after moulting shows that the 
“new external sacs appear in different positions to their predecessors. 
_Eyidently then the rootlets which communicated with the old 
external sacs do not regenerate new ones at once. 
It seems very likely that there has been an adjustment of the 
development period of the parasite to the time elapsing between 
Fic. 2. Thompsonia parasitic on Synalpheus showing mature external sacs 
springing from a tail-fan of the host. One is full of mature Cypris larvae: in the 
other the larvae have mostly escaped through the apical aperture ap. The root 
“system, 7, isshown up by the blackened yolk globules it contains. ch. the remnant 
of the chitinous envelope of the external sac. cl. enlargements of roots which 
will later develop into external sacs. Fixed in Flemming’s fluid. 
moults of the host. When the Alpheid under my observation 
cast its skin the external sacs contained advanced larvae, and 
I imagine that the disturbance connected with capture and 
change of conditions brought on the moult slightly before it would 
have normally occurred. If there is no such correspondence 
between the two periods, surely the moult,of the host will suspend 
the development of the parasite and interfere with the mechanism 
for securing the liberation of the larvae. 
_ The existence of crowded nuclei with the thinnest investment 
