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 Coutiere and Hafele 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 Hafele) 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 larvse 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 Coutiere and Hafele have 

 done. This point is specially emphasised by Coutiere. 



