24 
muscular layers of both even in immediate contact with each other. 
Those who have ever noticed the young stages of the copulatory organs 
in Polyclads, in form of strings of accumulated cells, can easily 
conceive how small a shifting in location would actually be needed 
to bring about a fusion. The alimentary system in Polyclads presents 
splendid examples of fusions of epithelial layers on a large scale. 
In several families we simultaneously find species with only a 
branched intestinal system and species with richly anastomosing 
gut-diverticula. In Discocelides Langi most of the intestinal coeca 
simply form branches, but in the anterior and posterior parts of 
the body they always anastomose (Bock, 1913, p. 81). In Styl- 
ochoplana gracilis Heath and McGregor, to give an example from 
the same genus of Polyclads, the intestinal apparatus anastomoses 
freely, while in most other Stylochoplana-speciQS it only branches. 
I have merely used these various examples in order to demon- 
strate the lability that exists in Polyclads. The male and female 
apertures in Polyclads might likewise sometimes fuse into a single 
one. This is the case, for instance, in Stylochoplana maculata 
Quatrefages, while other species of Stylochoplana have separate 
genital openings. The best example that I have seen of fusion of 
the distal parts of the male and female efferent duets is, however, 
in a new P^nceZ/d-genus, Thalattoplana, from the Bonin Islands 
in the Pacific. 
I cannot see any real obstacle for the assumption that the 
ductus vaginalis has originated from the duet belonging to Lang’s 
glandular vesicle. An additional reason for such a conclusion is 
found in Discocelides Langi Brgdl., where a canal connects the duet of 
Lang’s glandular vesicle with the distal part of the vagina. I do 
not intend to urge Discocelides as the original type from which 
Polyclads provided with a ductus vaginalis have developed. The 
case mentioned indicates best how various conditions may oceur, 
and how one may be entitled to the reasoning given above. 
Therefore, I must draw the obvious conclusion that the ductus 
vaginalis corresponds to the duet leading to Lang’s glandular vesicle 
and genetically has been derived from this duet. We shall no doubt 
also find that there are reasons to believe that such a change into 
a ductus vaginalis has taken place on its own accord in various 
cases, in Lepioplanidae as well as in Stylochidae. 
