6 Laidlaw, Classification of the Poly clad Turbellaria. 



ship to other Polyclads on our present knowledge of the 

 group. Certain other genera, such as Imogene, Diplonchus 

 ('84), are not sufficiently well known, and are also left out 

 of consideration. Polyporus ('98), described from an 

 immature specimen, is not included in the list, but is 

 perhaps allied to the Stylochidse. 



I offer this revision with some diffidence, the more so 

 that my grouping of the genera does not coincide with 

 that given by Lang in his monograph ('84). The genera 

 most affected belong, however, to his two families Plano- 

 ceridae and Leptoplanid^, and, as I have already hinted 

 (:03(5'), the discovery of such a genus as Disparoplana must 

 needs modify the definition of these families. As the 

 most important character used in my diagnosis, I have 

 taken the condition of the prostate gland ; whether pro- 

 vided with a duct of its own, or merely surrounding the 

 vesicular duct, or absent altogether. The last condition 

 is, I think, obviously a secondary one. Which of the first 

 two is the more primitive condition it is hard to say, 

 though the fact that the prostate with its own duct is 

 found in the Acotylea, in practically all the Cotylea, and 

 in such a genus as Bergendalia, indicates that it is, at any 

 rate, an ancient character. 



Explanation of Lettering in the Figures. 



a.m. antrum masculinum. pr.c. cavity of prostate. 



c. chitinous spines. p^-gl- prostatic glands. 



d.e. 



ductus ejaculatoriuy. 



sh.gl. 



shell-glands. 



m.pr. 



, muscular wall of the 

 prostate. 



St. 



stylet of penis. 



o.s. 



ouler muscular sheath. 



ut. 



uterus. 



P- 



penis. 



va. 



vagina. 



p.s. 



penis sheath. 



v.d. 



vas differens. 



pr. 



prostate. 



v.s. 



vesicula seminalis. 



