On the Rhizocephalan Genus Thompsonia, etc. 17 
no definite mantle cavity between the mantle and the visceral mass, 
for the latter fills up all the internal space. The so-called mantle 
cavities which are described by Coutiére and Hifele and figured by the 
latter author are, in my opinion, merely artifacts. In many of my own 
sections the shrinkage of the visceral mass due to unequal fixation has 
been followed by the appearance of a space between the former organ 
and the mantle, but in others the preservation is sufficiently good to 
show the true state of affairs. The thick external cuticle of the 
mantle prevents the proper penetration of reagents and on this account 
it is easy to make mistakes. 
The mantle consists, in my material, of a syncytium in which the 
nuclei are mainly concentrated on the external and internal borders. 
This is due to the fact that there is secretion of chitin on these two sur- 
faces forming an outer and inner investment of the mantle. The nuclei 
of the outer layer are more crowded and numerous in correlation with 
the greater thickness of the chitin here. The chitin of the internal 
boundary is exceedingly delicate, but it is only the presence of such a 
structure which enables us to homologise the outer part of the external 
sac with the mantle,* in the absence of a mantle cavity. Between 
the two thickly nucleated layers there is a mesh-work of protoplasm 
with a few scattered nuclei. 
The visceral mass consists of an investing layer of vacuolated tissue 
with scattered nuclei (=innere Gewebeschicht of Hifele) which does 
not appear to take part in the secretion of the inner layer of chitin 
(vac.) and a central mass, densely packed with nuclei, in which the ova 
arise (ov.). This central mass was called the ovary by Hafele. 
But distally to the ovary there is a second region where proliferation 
of nuclei takes place, but no egg cells arise. This is described by Hafele 
as a testis because the cells are rather similar to the spermatogonia of 
other Rhizocephala and many of them are in course of division. No 
spermatozoa are seen at any stage examined and an inspection of 
Hafele’s figures does not inspire much confidence in his conclusion that 
Thompsonia is hermaphrodite. The active division of the nuclei in 
this region is accounted for by its apical position, which makes it a 
centre of proliferation. Growth proceeds throughout the development 
of the ova, until the larve are ready to hatch, and Hafele states that this 
organ has disappeared by that time. Certain large cells, with nuclei 
four to five times as great as those of the surrounding cells, seem to 
me to be clearly egg cells rather retarded in development, and their 
occurrence here shows that the apical portion of the visceral mass is not 
substantially different from the rest, but contains a larger amount of 
embryonic tissue. 
*The roots, mantle, and visceral mass of Thompsonia form a syncytium without any cell 
boundaries whatever, and it is not correct to speak of layers of cells, as Coutiére and Hiifele have 
done. This point is specially emphasised by Coutiére. 
