III. 
Polyclads living together with the Pagurid Petrochirus 
californiensis Boucrez. 
Among the material brought home from Dr. Mortensen’s Pacific- 
expedition are three specimens of a small Prosthiostomid which in 
various ways differs from the typical organization of Prosthiostomum. 
There are, as a matter of faet, sufficient reasons for describing it 
as a new genus. Its biology, moreover, presents a particularly 
interesting feature. In both localities mentioned below the genus 
in question was collected in shells inhabited by the Pagurid 
Petrochirus californiensis Boucrez. Several specimens of an Aco- 
tylean Polyclad, Emprosthopharynx opisthoporus Bock 1913, were 
also found together with this Pagurid. Thus we are here introduced 
to new cases of Polyclads associated with other animals. Both of 
these Polyclad-genera are, consequently, carried around by their 
host, whose meal they probably also share. Without doubt the 
species also enjoys splendid protection by living in the shells in¬ 
habited by Pagurids. This case shows the particular tendency in 
Polyclads to crawl into cavities, etc. in order to get away from the 
light and to seek protection. It is a well-known faet that specimens 
of Leptoplanidae and Prosthiostomidae are found in the daytime 
underneath stones, between the roots of Laminarians etc., to use 
only a few examples. Latocestiis shows special skili in crawling 
into cavities and channels of Lithothamnions, a circumstance that 
I have had the opportunity to observe on the South Sea coral 
islands. In this way they at times succeed in balfling even the 
most earnest collector. Its negative phototropism can undoubtedly 
be regarded as an excellent means of avoiding danger, that should 
not be underestimated. 
