548 PROCEEDINGS OF THE NATIONAL MUSEUM vol. io8 



the proximal end of the prostatic vesicle is found the small, muscular 

 seminal vesicle from which the ejaculatory duct proceeds, passing 

 above the prostatic vesicle and continuing along the posterior side of 

 the latter and dorsal to the penis papilla to open just behind the male 

 gonopore or perhaps, one should say, in common with it. This is 

 certainly a very strange course for an ejaculatory duct; one expects it 

 to enter the base of the penis papilla and this is the case in other species 

 of Plehnia. However, the course described appeared clearly indicated 

 in the sagittal series mentioned; it could not be clearly made out in 

 the other series but neither could any entrance into the penis papilla 

 be found. The ejaculatory duct is composed chiefly of a thin muscular 

 wall of longitudinal fibers. 



The female apparatus is very similar to that of Plehnia arctica 

 (Plehn, 1896). The female gonopore is found shortly behind the 

 exit of the ejaculatory duct. It leads into a long vagina much 

 slanted forward and paralleling the ejaculatory duct. About at 

 the level of the proximal end of the prostatic vesicle the vagina 

 makes a backward curve and after receiving the common oviduct 

 proceeds posteriorly parallel to the vagina as a long duct that ter- 

 minates in a small oval Lang's vesicle. The whole tract consists of a 

 cuboidal epithelium in which cell walls were missing and has but a 

 slight muscular investment. The large cloud of eosinophilous cement 

 glands, conspicuous in the whole mount, were in evidence in the 

 sections along the vagina but have been omitted from the figure. 



Differential diagnosis : Plehnia tropica differs from other eyeless 

 species of the genus in the course of the ejaculatory duct and the 

 sclerotization of the distal end of the penis papilla. 



Holotype: USNM 28641, one whole mount; also USNM 28685, 

 best set of sagittal sections (one slide). 



Remarks: This is the third eyeless species of Plehnia to be dis- 

 covered. The type of the genus, Plehnia arctica (Plehn, 1896), 

 the first example of an eyeless poly clad, came from Spitzbergen. 

 The second, Plehnia caeca Hyman (1953a) came from the California 

 coast at some depth and was also found to occur in a variant with 

 two cerebral groups of small eyes. Whether the association of the 

 present species with an alcyonacean is obligatory or accidental cannot 

 be stated but the large yolky eggs and the relatively large copulatory 

 apparatus indicate some tendency to parasitism. The total want 

 of eyes in an acotylean polyclad poses a systematic dilemma as the 

 present classification of the Acotylea is based upon eye arrangement. 

 However, Bock (1913) satisfactorily placed Plehnia among the 

 Craspedommata and established the family Plehniidae. 



