On the Rhizocephalan Genus Thompsonia, etc. 31 
SUMMARY. 
(1) The root system of Thompsonia is continuous throughout the 
body of the host, whatever the number or stage of development of the 
external sacs may be. 
(2) Where they penetrate the appendages the roots contain fewer 
yolk globules and the lacunar space is filled with germ cells. Besides 
the external sacs, there are a number of terminal swellings in the tissue 
of the appendages. These are similar in structure to the external 
sacs and become external at a subsequent moult of the host. 
(3) The external sacs consist of a mantle with an external and in- 
ternal cuticular investment and a visceral mass mainly occupied by the 
ovary. No mantle cavity is found between the mantle and visceral 
mass. 
(4) There is no testis, development being in all probability partheno- 
genetic. The egg is lightly yolked and gastrulation takes place by 
epibole. The Nauplius stage is omitted from the life history, the young 
hatching at the Cypris stage. 
(5) During development the visceral mass disintegrates so that at 
the time of hatching the mantle contains a great number of Cypris 
larvee ready to emerge. An apical perforation is made in the mantle, 
and on the moult of the cuticle a way is thus opened to the exterior for 
the larve. 
(6) The escape of the larve is contemporaneous with or soon fol- 
lowed by a moult of the host. The empty shells of external sacs are 
carried away with the cast skin, and the terminal swellings of the root 
system emerge as a new crop of external sacs. 
(7) Development of the germ cells in the lacunar tissue of the roots 
may take place in situ as well as in the external sacs, but it does not 
apparently proceed very far. 
(8) The large number of external sacs in both Thompsonia and 
Peltogaster socialis is accounted for by a process of internal budding 
from a single original larva. 
(9) Thompsonia is not a primitive Rhizocephalan, but a very 
specialised form. 
(10) The genus Thylacoplethus Coutiére is synonymous with Thomp- 
sonia Kossmann. 
