Mr Potts, Thompsonia, a little known Crustacean Parasite. 455 
_ This conclusion was reached first after an examination of the 
abdominal appendages of an infected Synalpheus. These were 
snipped off from the living animal and examined under the low 
power of a microscope. Each flattened ramus of the appendage 
‘was seen to contain at least one slender root strand of a slightly 
greyish colour (owing to the doubly refringent yolk granules), 
giving off a secondary root to each external sac and generally 
a number of others which end in bulbous enlargements within 
the tissues of the host. The significance of these last will be 
explained later. 
_ Dissection of the trunk of the Alpheid also revealed the exist- 
ence of a network of slender grey roots in the neighbourhood of 
the nerve-cord. The extent of the root system in the body of the 
host was best demonstrated in preserved material. The infected 
Alpheids were fixed both in corrosive sublimate with acetic 
acid, and Flemming’s fluid. The latter gave by far the most 
favourable results, as the yolk granules were blackened by the 
osmic acid of the fluid and it was thus possible to recognise the 
roots and trace their course with the greatest ease. The root 
system is diffuse and is best developed in the ventral body wall, 
and particularly round the nerve cord, but in the thorax it passes 
laterally and dorsally, and in all places gives off branches into 
the musculature. The alimentary canal is not affected, so. the 
root system is not really so widely distributed as that of Sacculina, 
though much more so than that of Peltogaster. From this 
central root system within the trunk there pass out into the 
walking legs, the abdominal swimmerets and the tail fans, lateral 
roots which together with their branches may be distinguished as 
the peripheral root system. The branches of the central system 
show a syncytial epithelium surrounding a central lacunar space, 
and containing large yolk granules: their function is nutritive. 
[he peripheral part of the system has a reproductive function 
on the other hand. The lacunar space is filled with small nuclei 
m scanty cytoplasm, destined to give rise to the reproductive 
sells which develop in the external sacs. The vitelline granules 
wre small and broken up, suggesting that katabolic metabolism 
prevails here over anabolic. In addition to transmitting nourish- 
ment to the existing external sacs these peripheral roots are 
charged with the function of replacing them at the proper time 
oy a new crop. 
. The external sacs are probably homologous with the visceral 
nass of the typical Rhizocephalan, but differ from this in the 
‘mplicity of their structure, a simplicity which is partly due to 
sheir large number and small size. Of the organs contained in 
whe visceral mass of the higher Rhizocephala, the nerve ganglion, 
eproductive ducts, muscular tissue, and probably too the testis, 
30—2 
