ON TURBELLARIA OF THE SCOTTISH NATIONAL ANTARCTIC EXPEDITION. 135 



The conical elevation mentioned above may represent an apical portion of the penis, 

 and the oval organ a combined bulbar portion and granule gland. A vesicula seminalis 

 appears to be awanting, its place being taken by the much-dilated vasa deferentia. 

 The dorsal diverticulum of the sheath cavity may have something to do with the 

 mechanics of protrusion of the bulbar part of the penis. 



The immature specimen has no male external opening, but in a mass of parenchym 

 cells below the anterior part of the pharyngeal sac there are two small cavities 

 connected with one another. (Plate fig. 3.) These probably represent the penis 

 sheath cavity and the granule gland (bulbar portion of penis). From the dorsal side of 

 the first a minute cavity arises which probably gives rise to the apical portion of the 

 penis. The vasa deferentia unite behind the granule gland without opening into it, 

 and they are traceable for a considerable distance as narrow, undilated canals. It is 

 remarkable that at this stage there is no communication between the various cavities 

 mentioned above and the external surface. 



Female Organs. The female opening in specimen (a) lies just in front of the sucker 

 and near the junction of the first and second thirds of body length. It leads by a 

 narrow canal into the widened shell-gland portion. The duct of the uterus is traceable 

 for a short distance as a solid mass of cells, but its cavity is not yet developed. Uteri 

 and oviducts cannot yet be made out, and the ovarian tissue is in an extremely 

 embryonic condition. In the immature specimen only the cavity of what is probably 

 the shell-gland portion is recognisable (Plate fig. 3), and like the male cavities it is cut 

 off from the external surface. 



Systematic. 



Until other species of this genus are found and described, it would be premature 

 to delimit the characters in the above description that are of generic from those of 

 specific importance. We therefore confine ourselves to designating N. orcadensis as 

 type of the genus, referring to the above description for details, and summarising the 

 outstanding characters as follows : 



Slender nuchal tentacles containing eyes but no gut branches ; large sucker situated 

 a little in front of the middle of the ventral surface ; mouth in front of brain ; pharynx 

 long, tubular ; pharyngeal sac with a backward extension above the sucker ; gut with 

 unpaired anterior and five pairs of lateral roots ; genital apparatus entirely beneath the 

 pharyngeal sac. 



In general structure Nuchenceros resembles the Euryleptid genus Oligocladus, 

 LANG (5), having like it the mouth in front of the brain, the pharynx long and tubular, 

 and the genital openings below the pharyngeal sac which extends backwards above the 

 sucker. But in Oligocladus the tentacles are of the marginal type containing gut 

 branches, while Nuchenceros has nuchal tentacles such as aie characteristic of the 

 Acotylean Planoceiidee. 



Again, according to our description, the male organs of Nuchenceros resemble those 



(ROY. soc. EDIN. TRANS., VOL. XLV., 825.) 



