78 
having attained an overwhelming' development. Reaching from the 
dorsal to the ventral muscularis, the ovaries have compressed the 
intestinal branches to flattened dorso-ventral pouches, approaching 
an appearance of septa between the gonads. This species was found 
free-living. Is the reason for this faet only the circumstance that 
ihe species just in this State carries out its reproductive duties 
and is an epoek in youth? The pharynx, also relatively more in- 
significant than in any other Polyclad, denies it not. For the 
present the suggestions made have no great value but may serve 
only as an eventual working hypothesis. 
Returning to the two Polyclads associated with Petrochirus, we 
might further discuss the mutual relation between the guests and 
the host. It is not very likely that the intruders prove injurious 
to the host itself. My general knowledge of the feeding habits of 
the Polyclads seems to speak against such an assumption, and the 
sucking power of the pharynx is scarcely able to get ihe upper hånd 
over the cuticle-protected tail-end of the host. Only if this is 
occasionally damaged could the Polyclad be expected to go to an 
offensive. Bearing upon this subject, there is mentioned a case in 
the literature. Lesson (1830) describes a Planaria velellae [see 
Lang 1884, page 607; according to v. Graff (1892) identical with 
the pelagical Planocera pellucida] and States that „cette planaire 
s’attache au vélelles, dont elle dévore la partie charnue“. Also 
Graeffe has found it on Velella (v. Graff 1903). Cotyleans may 
often damage Ascidians (e. g. Crozier 1917 on Pseudoceros). They 
appear then as robbers (cf. v. Graff 1903, p. 40). And the extreme 
robber nature of the Polyclads, of which I have personally numerous 
examples, cannot be doubted. This makes it not unlikely that the 
intruders would play havoc among egg collections of the host. In 
several instances, in yet undescribed Polyclads, I have found the 
alimentary system abounding with large eggs of other animals, e. g. 
Notoplana species in Japan. The occurrence of large quantities 
of egg-material in the intestine of Emprosthopharynx opisthoporiis 
has already been related. This species is perhaps not always an 
innocent guest. 
The purpose of the stay is to get shelter. Negative photo- 
tropism is a common feature in the Polyclads. A search for conceal- 
* 
ment might have been the driving force when they took up their 
