Fapers from Dr. Th. Mortensen’s Pacific Expedition 
1914—16. 
XXV. 
Planarians. Parts I—III. 
By 
Dr. Sixten Bock, Uppsala. 
(Willi Plates I, II and II a,) 
I. Ceratoplana, a Polyclad genus living beneath Colobocentrotus. 
II. A Stylochidean Polyclad with ductus genito-intestinalis. 
III. Polyclads living together with Pagurids. 
The three parts of this paper were intended by the author to form 
three separate papers. For editorial reasons it has been necessary 
to let them appear as chapters only of one common paper. This 
explains that the three parts are not logically connected, a faet for 
which the author is not to be held responsible. For the same reasons 
the explanations of the plates and the lists of literature have been 
put together, at the end of the paper. 
I. 
Ceratoplana, a Polyclad genus living beneath 
Colobocentrotus, and a 
Synopsis of the ductus vaginalis in Polyclads. 
(Plate I.) 
The splendid collection of Turbellaria which my friend Dr. 
Th. Mortensen brought home from his Pacific-Expedition in 
1914—16 has been sent to me for study, Among the material 
are several Polyclad-specimens, which, on account of the cireum- 
stances connected with their discovery, particularly caught Dr. 
Mortensen’s attention. They were found underneath sea-urchins 
of the species Colobocentrotus atratus (L) at Hilo, Hawaii. During the 
expedition to the Kei Islands Dr. Mortensen also found on Kra- 
katau a couple of Polyclad-specimens underneath the same species 
of sea-urchins. As these finds also in anatomical respect are of 
considerable interest, it is with particular pleasure that I carry out 
Dr. Mortensen’s wish to give a special report on them. 
Vidensk. Medd. fra Dansk naturh Foren. Bd. 79, 
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