14 



Papers from the Department of Marine Biology. 



The roots of the peripheral part of the system (text-figure GB) 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. 



FIG. 5. 



Thompsonia parasitic on Synalpheus 

 brucei, 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, r, 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. 



ap 



It will be well now to consider the terminal swellings of the rootlets 

 (text-figure 4s). 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 



