On the Rhizocephalan Genus Thompsonia, etc. 13 
within the broad area of the pleuron. One of these branches has given 
rise to an external sac, of which the peduncle (ped.) alone is shown, but 
sacs occur only rarely in this position. 
Similar wide extension of the system occurs when the tail fans and 
telson are penetrated, as is shown in text-figure 5. The single root 
whose course is followed divides into two, each of which gives rise to 
numerous short rootlets and finally anastomoses with its fellow. The 
rootlets usually end in club-shaped enlargements, but two have fully 
formed external sacs. 
Fra. 4. 
A. Pleuron of abdominal appendage of Synalpheus. brucei, showing root system of 
Thompsonia. r.nut., a number of roots like those of the interior of the 
trunk with large yolk globules; r.rep., roots of the peripheral system with 
much smaller yolk globules. These give off one branch to an external sac, 
which, however, has been detached, leaving the peduncle, ped., another 
process, 7b., which will shortly become an external sac, probably at the 
next moult, and numerous others which are not at all developed as yet. 
Fixed in Flemming’s fluid. X75. 
B. Internal bud of Thompsonia. This is ib. of fig. A more highly magnified. 450. 
The histology of the internal roots (text-figure 64) is similar to that 
described in Sacculina, with some modification perhaps due to the fact 
that they are thinner than those of the last-named form. In diameter 
they vary from 10to20 yu. There is a very thin cuticular investment 
covering a syncytial external layer, which contains a number of nuclei 
dispersed at irregular intervals. In the middle is a lacunar space, 
but I have not been able to demonstrate a lacunar tissue of stellate 
cells, such as is found in Sacculina. It is possible that this may be 
absent owing to the thinness of the roots. The large and numerous 
yolk globules are placed in the syncytial layer. 
