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. 



r.nut 



r.rep. 



ib 



ped 



FIG. 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, ib., 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. X450. 



The histology of the internal roots (text-figure GA) 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 10 to 20 M. 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. 



