14 Papers from the Department of Marine Biology. 
The roots of the peripheral part of the system (text-figure 6B) show 
two well-marked differences. The first of these I have already men- 
tioned, namely, the smaller size of the yolk globules indicating that 
consumption rather than accumulation of food stores is the physio- 
logical condition in this region. The second is the occupation of the 
lacunar space by masses of small rounded nuclei with a scanty cyto- 
plasmic investment. The external layer is, on the other hand, free 
from nuclei, which thus appear to have migrated internally. The 
origin of this lacunar tissue must be looked for in the external syn- 
cytium. Its function is undoubtedly reproductive; it accumulates 
particularly as the end of the rootlets which give rise to external sacs. 
Pia. 5. 
Thompsonia parasitic on Synalpheus 
brucei, showing mature external sacs 
springing from a tail fan of the host. 
One is full of mature Cypris larve. 
In the other the larve have mostly 
escaped through the apical aperture, 
ap. The root system, 7, is remark- 
able for the large number of internal 
buds, cl., none of which are advanced 
in development. At an. anastomosis 
of two branches may be seen. The 
external sacs have recently under- 
gone a moult and the remnant of the 
chitinous envelope, ch., is seen around 
the peduncle. Fixed in Flemming’s 
fluid. X35. 
It will be well now to consider the terminal swellings of the rootlets 
(text-figure 48). These are formed by the development of the exter- 
nal syncytial layer as well as the enlargement of the lacuna within. The 
external layer increases in thickness and the nuclei become much more 
numerous, migrating superficially, taking up a position with their long 
axes at right angles to the border. In fact, the tissue tends to assume 
a regular epithelial structure. Internally the protoplasm contains 
numerous yolk globules of various sizes and nuclei are rare and sparse. 
The interior lacuna is occupied by a dense mass of small rounded 
cellules of equal size which will give rise to the egg cells and nutritive 
cells of the external sac. 
The whole structure is in fact a developing external sac. By the 
increase in size of the structure it comes into close contact with the 
chitinous cuticle, so that if the Alpheid were to moult it would emerge to 
the exterior. It is well known, of course, that Sacculina prevents its 
