BRITISH MARINE TURBELLARIA. 499 



1845. PoLYCELis LiEviGATUS, Quatrefages (18). 



1849. Planaria flexilis, Thompson (25). 

 1853. „ „ Dalyell(29). 



1865 . Leptoplana tremellakis, Johnston (38) . 



1866. „ FLEXiLis, Ray Lankester (39). 

 1874. „ „ Mcintosh (45). 

 1886. „ TREMELLARis, Kochlcr (55). 

 1886. PoLYCELis LiEViGATUS, Koehler (55). 



Length 12 — 25 mm. Body delicate, of variable shape, more 

 or less elongate, broader in front than behind, the anterior margin 

 semicircular; young specimens, as Dalyell has remarked, have 

 the outline of a spherical triangle. The colour, if present, is 

 brown ; it is, however, extremely variable in amount and intensity : 

 it is due partly to parenchymatous pigment, partly to the gut 

 branches. In the middle line, not far from the hinder end, 

 are two white areas; the foremost represents the male copula- 

 tory organ, the one behind it the " shell-gland.^' Between 

 these and the brain a brown median area, surrounded by a 

 clear whitish space, represents the main-gut and the uterus 

 outside it. A V-shaped spot leading to the male pore is due to 

 the underlying vasa defereutia. The ovaries appear (especially 

 on a black ground) as white dots. From the white ventral 

 surface the plaited pharynx and genital organs may be seen. 

 Active swimming movements are produced by the expanded 

 edges of the anterior end of the body. Between the male and 

 female genital apertures is a depression, the lips of which are 

 strongly muscular, and constitute a ''sucker." The mouth 

 is in front of the centre. It leads into the strongly puckered 

 pharynx, lying in its sheath. From this the main-gut arises, 

 and runs forwards towards the brain, and backwards to the 

 commencement of the posterior third, giving off as it does so 

 the numerous lateral branches, which in turn subdivide and 

 end in fine cseca along the margin. The brain is distinctly 

 bilobed. The lobes are oval, their long axes parallel to one 

 another and to that of the body. Five anterior pairs of nerves 

 supply the region in front and at the sides of the brain, 

 and two lateral ones the rest of the body. The eyes 



