6 ARKIV FÖR ZOOLOGI. BAND 14. NIO 13. 



I may at present leave a detailed description of it out of 

 this account, but I must point out that the pharyngeal fold 

 possesses a central lamella of circular muscle fibres. The 

 Acotylea, which are all provided with a orinkled (krausen- 

 förmig) pharynx, have such a central layer and I must think 

 that this pharynx-type is the primitive one for the Polyclads. 

 The central muscle layer in the pharynx is mentioned b}^ 

 Lang only for the Pseudoceiidce and the genus Eurylepta 

 amongst the Cotylea. It may very probably also occur in 

 the primitive cotylean genus Anonymus, but Lang, its only 

 investigator, does not, however, mention the histological 

 structure of its pharynx. As I can ascertain from my own 

 researches, it is also present in the pharynx of some new 

 Pericelideans but Meixner (1907, p. 475) failed to observe it 

 in P ericelis Beyerlana (Collingwood) Laidlaw. The pharynx of 

 Pericelis is also »krausenförmig>>. This central muscle lamella 

 is lacking in the bell- and tube-shaped pharynx of the other 

 Cotyleans, this being, in my opinion, a secondary feature. It 

 may not have been necessary to keep it when the pharynx 

 has acquired the bell- or tube-shape, as these shapes make 

 it more easy for the pharynx to be protruded thiough the 

 mouth and the transverse muscle layers of the external and 

 interna! wall of the pharyngeal fold being sufficiently strong 

 to narrow the bell or the tube; these muscle layers are 

 accordingly very well developed in these pharynx-types. 



The internal mouth lies a little behind the level of the 

 external one. The main gut is wide and can be followed 

 behind the sucker region. There is no unpaired gut branch 

 above the brain. Its anterior end just reaches the brain 

 but does not pass över it. The gut branches are few and 

 are not beadlike. I have not observed them to anastomose. 



The generative organs. 



The testes are uniformly distributed in the ventral zone 

 of the body outside the pharynx and the copulatory appa- 

 ratuses. They are comparatively large, 100 — 150 {j., and, as 

 they are not closely packed, their number is not excessive. 

 Their membrana propria is well defined and thick. 



The male copulatory apparatus (text fig. 3) is very unlike 

 that usually met with in the Cotylea. There is a large thin- 

 walled vesicle lying close below the pharyngeal sac and 



