163 
together of organs of different degree of specialization is a char- 
acteristic feature of the Stylochidæ. It is apparent that this will 
hamper very much the efforts to bring the genera in a proper 
sequence in this family; this so much more the case as it is many 
a time difficult to decide whether a simple structure means a 
primitive condition or is the result of a regressive evolution. A 
simple organization in one respect is often combined with far 
advanced specialization in regard to some other organs. I have 
particularly laid stress upon the genital system and especially the 
copulatory organs when it comes to estimation of relationship and 
ranking of the genera. But the male and female organs do not 
follow each other in specialization. A rather striking case is pre- 
sented by the genus Stylochiis. The prostatic vesicle attains here 
its most complicated structure, while at the same time the female 
apparatus has the simplest structure in the whole family. However 
the development of body, tentacles, nervous system, and alimentary 
system, especially pharynx, has gone very far. The simplest type 
of male apparatus is met with in Enterogonia, but this condition 
is decidedly not primitive, at least as far as it concerns the pro¬ 
static vesicle. 
If we then take the male copulatory organ as a subject for a 
comparative study, it is evident that none of the Stylochids yet 
described presents a real primitive type from which the other may 
have developed. According to the view which I accept, the original 
type for the male copulatory organ of Stylochidæ must have been 
a kind of glandular organ with muscular envelope, in possession 
of a free tip projecting into a pocket or antrum, which opened on 
the ventral side of the body behind the pharynx. This organ was 
the prototype of the prostate -f- penis -f- antrum. The seminal 
canals have later established connection with this glandular organ. 
At that time the penis was nothing but the very apex of this 
„prostatic“ organ. The separation of the prostatic vesicle from the 
penis, which in many cases has gone so far that it appears as an 
independent organ, stout and fleshy, or small and armed with a 
stylet, is thus a secondary feature. The pocket surrounding the 
tip of the glandular organ has been changed to an antrum mas- 
culinum. It was first small and formed originally by a simple 
invagination of the epidermis. The penis-sheath is a secondary 
11 * 
