60 
* 
separating the species from the genus Prosthiostornum. On the 
oiher hånd, I must attach greater importance to the dissimilarity 
in the genital organs and especially to the faet that the pair of 
accessory vesicles in the male copulatory apparatus has an organi- 
zation dilferent from that indicated in Prosthiostornum. In so 
doing I wish to refer the reader to the above description and call 
to mind the faet that these organs show absolute conformity in all 
the Prosthiostomum-s^tcxQS that have been more closely examined. 
This is also the case in the many new species of Prosthiostornum 
from the Pacific which I shall describe elsewhere. It is only ne- 
cessary to add here that Enchiridium shows an entirely different 
course of development as regards these vesicles. They have here, 
as a matter of faet, become one single organ through having a joint 
muscular envelope, but Euprosthiostomum shows its relationsship to 
the Prosthiostomum-\y^Q through the spherical shape of this vesicle, 
as it were, and through the condition of the efferent duets. 
The absence of an unpaired intestinal branch over the pharynx 
in this new genus reminds one of a similar differentiation in the 
genus Stylostomum of the family Euryleptidae. Lang (1884, p, 140) 
describes the latter feature as follows: „In einem Punkte weicht 
der Gastrovascularapparat von Stylostomum von der aller iibrigen 
Polycladen ab. Vom vorderen Ende des Hauptdarmes bis zum Gehirn, 
also in der Gegend des Pharynx und des mannlichen Begattungs- 
apparates existiert kein vorderer medianer Darmast“. The intestinal 
branch over the brain in Euprosthiostomum arises, as a matter of 
faet, in the same way as in Stylostomum', i. e. from the anastomosis 
between the most anterior pair of intestinal branches. Thus we 
have here a case of congruent development, dependent on the 
same factor: the remarkable inerease in size of the pharyngeal 
pocket. In this connection I might call to mind the case of an- 
other Euryleptid, Åceros baeckstroemi Bock 1923 a, from Juan Fer- 
nandez, which has evaded the difficulty arisen through the inereased 
size of the pharyngeal pocket by moving the unpaired anterior 
intestinal diverticle to the side of the pharynx. 
Besides the differentiations mentioned, there are also others, 
such as, the reduction in the number of genital giands and their 
absence in the most anterior and most posterior parts of the body, 
the organization of the seminal vesicle, the forward curve of the 
