2 
Colobocentrotus lives in places that are particularly exposed to 
the action of the surf. From my own stay on the Bonin Islands, 
I have personal experience as to the ability of this sea-urchin to 
withstand even very strong breakers without being torn away. Such 
localities are, however, hardly suitable for a sensitive Polyclad-fauna. 
Under such circumstances, the finding of the Polyclads underneath 
Colobocentrotus can hardly be a matter of chance, especially as al- 
together no less than six specimens were collected together with 
this sea-urchin, and two so widely separated localities are concerned 
as Hawaii and Krakatau in the Sunda Strait. In Dr. Mortensen’s 
large collection of Polyclads from the Pacific and the Malayan archi- 
pelago there is no other representative of this new genus. 
This Polyclad can at the most be considered an „epok“, taken 
in the meaning that Cramer has given the term, or possibly as 
a commensal, and is thus no true parasite. Ceratoplana seeks 
first of all protection underneath Colobocentrotus, which cannot be 
easily moved by the breakers. It probably also shares the meal 
with its „host“, snatching the food-remnants. 
The Gommensalism of Polyclads. 
Any absolutely certain cases of pure parasitism among Polyclads 
are to this date unknown. The Polyclads that have been found 
on other animals, must be considered either as „ep6ks“ or as 
commensals, unless they are robbers attacking the soft parts of 
their „hosts“. In regard to the finds, when only a single speci- 
men has been detected on an Echinoderm, in a shell, or in an 
Ascidian, it appears safer to consider them with Lang (1884, p. 
559, regarding Prostheceraeus Giesbrechti) as temporary guests until 
similar finds have been repeated. With this reservation I shall give 
a report on the Polyclads that have been found associated with 
other Evertebrates. However, I shall entirely pass by such cases 
as, for instance, 5^/oc/t«s-species and Discoplana subviridis (Plehn), 
that live among branches of living Madreporarians (observed by me 
on the islands of Bonin and Gilbert in the Pacific), Notoplana ato- 
mata (O. F. Miill) and Leptoplana tremellaris (O. F. Miill), that occur 
in masses on My^z7«s-colonies (on the west coast of Scandinavia), 
Prostheceraeus vittatus (Montagu) and Stylostomum ellipse (Dalyell)* 
on Ciona intestinalis-colonles, and Stylochus on young oyster-colonies,. 
