17 
ihe Wall, as in this case, is so simple in its organization and shows 
so little differentiation. This similarity in the structure can easily 
have been acquired later, when the ductus vaginalis assumed the 
fun etion of a vagina; i. e. to receive the male copulatory organ. 
The epithelium diminishes in thickness in the genital opening 
and has in the antrum, as well as in the ductus vaginalis, only 
about one-half the stoutness of the body epithelium. Both tubes 
lack giand cells. The ciliated covering is feebly developed in the 
antrum, but slightly better in the ductus vaginalis. The thick base¬ 
ment membrane of the body-wall diminishes quickly in stoutness 
Text-fig. 7. Ceratoplana colobocentroti var. hawaiiensis. Diagrammatic lateral view of 
genital apparatus 53 x. Explanation of letters in text-fig. 6, 
inside the genital opening and becomes already in the middle part 
of the antrum as thin as a line. The muscularis of the wall of 
the antrum consists almost exelusively of circular muscle fibres and 
reaches twice the height of the epithelium. In the strict medial, 
sagittal sections the muscular layer looks as though it were the 
direct continuation of the circular muscle layer of the body-wall. 
The vagina is very short, and, running forwards and upwards, 
describes a slight curve backwards. It is already at its outer 
beginning a great deal wider than the antrum femininum, but 
narrows in its upper part, which lacks shell giand openings. The 
median uterine duet (mu), which is of considerable length, is di- 
rected frontwards, and is, strangely enough, not any narrower than 
pars interna vaginae. From the entrance of the median uterine 
•duet the ductus vaginalis forms the direct continuation of the va- 
Vidensk. Medd. fra Dansk naturh. Foren. Bd. 79. 2 
