POLYCLAD FLATWORMS HYIVIAN 555 



It is puzzling that the collections contain so many juvenile lep- 

 toplanids and so few adult ones. It further appears that a number 

 of distinct species of Leptoplanidae must be present around the 

 Palau Islands. 



Family Planoceridae Lang, 1884 



Aquaplana pacifica, new species 



Figures 4a-c; 5a 



Material: One specimen collected by the Stanford team at Sta, 

 220, Palau Islands, southeastern end of Koror Island in Oj^ster Pass, 

 near east entrance of Iwayama Bay, Oct. 12, 1955. 



General characters: The specimen is of broadly oval form (fig. 

 4a), measuring 14 by 11 mm., and of a transparent texture but pep- 

 pered vaih. minute brown dots that are very abundant over the copu- 

 latory region but diminished over the pharyngeal area. There are 

 two tentacles, contracted to rounded form, that contain no eyes but 

 are also peppered with brown dots. The numerous eyes occur in 

 paired tentacular clusters and in a loose cerebral group that is not 

 very definitely paired (fig. 46). From the central ruffled pharynx 

 narrow intestinal branches radiate to the periphery, and a main in- 

 testinal branch extends anteriorly, subdividing into three. The uteri, 

 stuffed with eggs, begin just behind the tentacles and curve posteriorly 

 lateral to the pharynx, converging to the female copulatory apparatus. 

 The copulatory apparatuses are found somewhat posterior to the 

 pharynx, and because of the transparency of the animal show many 

 details prior to sectioning (fig. 4c). 



Copulatory apparatus : The copulatory region was removed and 

 sectioned sagittally, and the apparatuses are shown in sagittal view 

 in figure 5a. All parts of the male apparatus are bound within the 

 same muscular sheath (fig. 4c). The spermiducal vesicles (expanded 

 sperm ducts) approach the male apparatus from behind, and at its 

 sides acquire a thick coat of circular fibers, thus becoming spherical 

 spermiducal bulbs. Their muscle fibers are continuous with those of 

 the rest of the male apparatus (fig. 4c). Prostatic vesicle and cirrus 

 sac form one continuous structiu'e with a continuous muscle coat. 

 The anterior end of this structure constitutes the prostatic vesicle, 

 not demarcated from the cirrus sac. It has a thick muscular coat of 

 fibers that mostly parallel its external contour and the relatively 

 small pyriform cavity is fined by ridges of glandular epithelium, 

 filled with eosinophilous granules. The prostatic duct continues pos- 

 teriorly as the ejaculatory duct, centrally placed in the cirrus sac. 

 This duct shortly receives the short common sperm duct into its 

 ventral side (fig. 5a), formed by the union of the two sperm ducts. 



