NO. 2 HYMAN : POLYCLAD FLATWORMS FROM THE GALAPAGOS 185 



The vesicle is covered externally with a well-developed muscle layer, 

 thicker dorsally than ventrally, and perforated dorsally by muscle strands 

 at right angles to its fibers. Distally the prostatic vesicle narrows to the 

 ejaculatory duct that traverses the center of the conical penis papilla and 

 opens at its tip. The penis papilla lies in a small male antrum that paral- 

 lels its contours and opens below by the male gonopore. As usual in acoty- 

 lean polyclads, the male apparatus is situated immediately behind the 

 pharynx, with the penis directed backward. 



The female apparatus lies close behind the male apparatus, with 

 a separate female gonopore located well behind the male gonopore. The 

 female gonopore leads into an exceptionally narrow, tubular female 

 antrum, from which the vagina proceeds anteriorly and then makes the 

 usual backward curve. The vagina is entered along its course by the 

 cement glands, which are thickest at the curve. Shortly after the curve, 

 the vagina receives the oviduct from below and then continues as the 

 duct of Lang's vesicle. This duct almost equals the vagina in length; it 

 gradually descends and opens into the long, oval Lang's vesicle. 



Differential characters. — Of the ten previously described species of 

 Latocestus, one, the type species atlanticus Plehn, 1896, is described as 

 devoid of marginal eyes, no doubt an error of observation. Four others, 

 pacificus Laidlaw, 1903, plehni Laidlaw, 1906, marginatus Meixner, 

 1907, and caribbeanus Prudhoe, 1944, have the marginal band of eyes 

 limited to the anterior part of the body margin. Of the five species with a 

 complete band of marginal eyes, ocellatus Marcus, 1947, differs from all 

 other species of Latocestus in possessing a common gonopore. The sexual 

 anatomy of L. maldivensis Laidlaw, 1902, and of argus Laidlaw, 1903, 

 has not been described ; but the arrangement of the frontal eyes in the 

 former differs altogether from that of the present species, and argus is 

 said to resemble atlanticus as regards sexual anatomy. L. atlanticus is 

 provided with pronounced spermiducal bulbs, that is, strong musculariza- 

 tions of the terminal parts of the sperm ducts ; and this is also the case in 

 /. viridis Bock, 1913. A lack of such muscularization appears not only 

 in the present species but also in L. marginatus and in L. whartoni 

 (Pearse), 1938; the latter, however, differs from galapagensis in the 

 short crescentic Lang's vesicle. A large elongated Lang's vesicle is 

 found in viridis and atlanticus as well as in galapagensis. 



Remarks. — This appears to be the most common polyclad of the 

 Galapagos Islands, as it was taken in three different locations on three 

 different expeditions, and in greater numbers than those of any of the 

 other species. The genus Latocestus in general is limited to tropical and 

 subtropical waters. 



