16 Papers from the Department of Marine Biology. 
the root system has had time to spread throughout the whole body 
of the host. From the fact that the external sacs appear at first only 
on abdominal and never on thoracic appendages alone, we may safely 
conclude that the internal stage of Thompsonia fixes and grows in the 
abdomen of the host, as is the case in Sacculina and probably the other 
members of the Rhizocephala. 
THE STRUCTURE OF THE EXTERNAL SACS. 
It has already been pointed out that the external sacs are rounded or 
cylindrical bodies attached to the host by a peduncle and varying in 
length from 0.5 to 1.0 mm. in the parasite of Synalpheus, attaining 
to 3 mm. in that of Thalamita. The smaller forms are immature, 
the larger are occupied by larve ready to hatch. The external sacs 
are probably homologous with those of the typical Rhizocephala, but 
differ from this in the simplicity of their structure, a simplicity which is 
due to their large number and small size. Of the organs contained in 
the visceral mass of Sacculina or Peltogaster, the nerve ganglion, repro- 
ductive ducts, muscular tissue, and probably too the testis, have been 
lost. 
Fig. 7.—Young external sac of Thompsonia; whole preparation to show general 
appearance of mantle and ovary. X60. 
Fic. 8.—Portion of transverse section of a more advanced external sac. 240. 
m., mantle; ch. ex., external layer of chitin; ch. in., internal layer; vac., 
vacuolated external layer of visceral mass; o., ovary, with ov., develop- 
ing eggs, and i.c., interstitial cells. 
Coutiére states that the external sacs of Thylacoplethus show the 
following typical Rhizocephalan structures: ‘‘Un manteau 4 double 
paroi, dans lequel est suspendue une masse viscérale et qui porte une 
ouverture cloacale.’”’ With this general summary I am in agreement. 
Hafele, on the other hand, does not appear to recognise any homology 
between the external layer of tissue (Aussere Gewebeschicht) and the 
mantle of other Rhizocephala, or between the internal body (internale 
Gewebeschicht and Ovarium) and the visceral mass. There is, however, 
