POLYCLAD FLATWORMS — ^HYMAN 591 



sperm ducts from the spermiducal vesicles into the seminal vesicle 

 could not be found. The ejaculatory duct issues ventrally from the 

 thinned distal end of the seminal vesicle, turns forward, and pursues 

 a sinuous course to the proximal end of the penis, accompanied above 

 by the duct of the more anterior accessory vesicle, below by the duct 

 of the more posterior one. The penis stylet as seen in the whole 

 specimen before sectioning is shown to the left of the posterior part of 

 figm*e 166. The male antrum is lined by an epithelium of tall, nar- 

 row cells, outside of which occur considerable layers of circular and 

 longitudinal muscles. These layers continue along the ventral 

 body wall to the female pore. 



The female gonopore leads by a short tubular female antrum into 

 a vaginal chamber lined by an epithelium of tall, narrow cells, with 

 very little muscular investment. This again leads by a short narrow 

 passage into a larger chamber, the glandular vagina, or cement pouch. 

 The cement pouch is lined by an epithelium of less narrowed cells, 

 penetrated by the outlets of the cement glands. The latter siuround 

 the cement pouch as a mass of eosinophilous granules in which cells 

 are not detectable. The cement pouch lacks musculature. From its 

 anterodorsal side the vagina continues as a tube that receives the 

 uteri about on a level with the posterior accessory vesicle of the male 

 system. 



Behind the female apparatus is seen the sucker, forming a rather 

 large deep pouch lined by an epithelium of exceedingly long narrow 

 cells underlain by a muscular investment about equal to that of 

 the ventral body wall with which it is continuous. 



Differential diagnosis: Marcus (1949, 1950, 1952) listed the 

 described species of Prosfhiostomum, with authors and references, and 

 in the 1952 article added one new species, making a total of 53 mem- 

 bers of the genus. Since then there have been described P. latocelis 

 Hjiann (1953a) and P. multicelis Hyman (1955a). As the members 

 of the genus are all very much alike, specific identification poses a 

 difficult problem. The main characters are the eye pattern and 

 number and details of the copulatory apparatuses. A few species 

 have a distinctive color pattern but most are white or pale. To de- 

 termine the status of the present specimens, all the original descrip- 

 tions were inspected (except those of Stimpson, 1857, probably un- 

 recognizable) but none could be found agreeing with theu characters. 

 In its small size at sexual maturity P. exigwum differs from most of 

 the described species. A pair of eyes set off from the cerebral clusters 

 occurs in P. siphun cuius, P. monosora, P. drygalskii, P. parvicelis, 

 P. sonorum, P. vulgare, P. delicatum, P. notoensis, and P. nozakensis, 

 but other details of eye pattern and number differ from those of P* 

 exiguum except in P. dnjgalskii and P. vulgare. In these two species 



