74 
1923. When I wrote my paper on this remarkable form, the de- 
scription of Cestoplana polypora was not yet published, and un- 
fortunately it remained unknown to me until 1924. Without 
presenting more facts in support, I would not then (1. c. page 31) 
give my explanation, but I emphasized strongly the nature of the 
giands carrying their secretion to the vesicles. „In all essential 
points the giands closely resemble the shell giands surrounding the 
vagina“. The derivation of the vesicles from a splitted or multi- 
plied first beginning of the female apparatus was less evident as they 
had globe shape, and, no analogous examples being recorded, this could 
consequently be doubted by the reader. Now in Cestoplana polypora 
the case is clear and convincing in itself, as the organs agree with 
one another. The multiplication of the female apparatus is there too 
apparent to be doubted. That there must exist connections with 
the uteri is beyond doubt, though not actually detected. Already 
in 1913 (page 249) I pointed out that Cestoplana in respect to 
male and female apparatus shows resemblance to Emprosthopharynx, 
as well as to the more primitive type among Leptoplanidae. As I 
have also pointed out, disagreements as well are present. A discussion 
as to eventual relationship of E mp r osthop hary nx to Cryptocelidae 
(Microcelis) was also carried out (1. c. pag. 166). I can hardly say 
that the knowledge of the Acotyleans has increased so much that 
we are yet able to trace the actual relationship of this aberrant 
genus to other families. 
The relation of the two Polyclads to their host. 
There remains to be taken into consideration the kind of asso¬ 
ciation that these Polyclads have with their host, Petrochirus cali- 
forniensis Boucrez. The time has passed when every animal 
found intimately associated with another and living on its surface 
was uncritically regarded as a real ectoparasite. What is then the 
meaning of parasitism? In 1906 Max Brown defined parasites as 
living organisms which for the purpose of obtaining food take up 
their abode, temporarily or permanently, on or within other living 
organisms. Though a general agreement is scarcely to be ex- 
pected in respect to such a vague category as parasitism, the 
restriction, for the purpose of obtaining food from the 
host, seems quite necessary. To the definition I would also add 
