592 PROCEEDINGS OF THE NATIONAL MTJSEXJM vol. los 



eye number and arrangement are identical with those of P. exiguum 

 and, in fact, for some time it was thought the present specimens 

 might be P. vulgare Kato (1938b), but the details of the copulatory 

 apparatus ai"e UTeconcilable between the two. P. exiguum is then 

 distinguished by the following combination of characters: small size 

 at sexual matm-ity, uniform pale coloration, eye pattern (fig. 166, c), 

 long cylindrical male antrum almost horizontally oriented, retort- 

 shaped seminal vesicle, issuance of ejaculatory duct posteriorly and 

 ventrally, and female apparatus of two successive chambers. 



Holotype: USNM 28668, the larger specimen as whole moimt. 

 The smaller specimen (USNM 28687) is deposited as sagittal serial 

 sections (one slide). 



Prosthiostomum griseuni, new species 



Figures 16d; 176 



Material: One specimen collected by D. Reish at Parry Island, 

 Eniwetok AtoU, on the lagoon side in September 1956. 



General characters: This is a very small worm, 4 mm. long, of 

 the slender shape characteristic of the genus (fig. 176). It is of a 

 uniform dark gray coloration. The eye pattern (fig. IQd) is very 

 unusual in the genus. The marginal eyes, relativelj^ few in number 

 and rather large, are not arranged in the usual marginal band but ai'e 

 scattered over the anterior end between the cerebral eyes and the 

 anterior margin. The cerebral e3^es occur in the usual two groups 

 of 8-10 eyes each. The tubular pharynx is partly protruded. The 

 sucker occurs unusually far posterior to the pharynx, about 0.8 mm. 

 behind the root of the latter. The specimen is juvenile with only the 

 begimiings of gonads and no indication of copulatory apparatuses. 

 It has such distinctive characters, however, that giving it a specific 

 name appears justified. 



Differential diagnosis: The scattering of the mai'ginal eyes over 

 the anterior end is seen in only one other of the 56 described species 

 of Prosthiostomum; namely, in P. latocelis Hyman (1953a). The 

 latter, however, is pale, mth numerous cerebral eyes, and with the 

 sucker close to the root of the pharynx. The present species is 

 readily distinguished by the gray color, scattering of the marginal 

 eyes over the anterior end, and posterior position of the sucker. 



Holotype: USNM 28669, a whole mount. 



Remarks: Kato (1938a) created the genus Amakusaplana for a 

 prosthostomid identical with Prosthiostomum except that all eyes are 

 scattered over the anterior end and a sucker appears wanting. The 

 species Prosthiostomum latocelis and P. griseum constitute forms inter- 

 mediate between t3^pical Prosthiostomum and Amakusaplana in that 

 their marginal eyes are scattered but their cerebral eyes remain in 



