248 Mr. Darwin on several Planaripe, 



mouth-sucker and its external orifice are situated. The three main 

 branches receive the moss-like subdivision of the intestinal cavity, 

 which reach all round nearly to the margin of the body. The main, 

 medial, intestinal cavity ends at the anterior extremity in a small, 

 opake, wedge-formed mass ; on each side of which, nearly on the 

 dorsal surface, a black ocellus is situated. Between the lateral 

 branches on each side of the medial cavity, seven or eight internal 

 spherical cavities lie, including opake balls, which I presume are 

 immature ova ; the anterior ones were most developed : they were 

 not present in the smaller specimens, or in all the full-grown ones. 

 I was unable to discover any genital orifice, though no doubt one 

 or two exist : near the posterior exti'emity (at B) there was a 

 colourless space, but I could not see any orifice. Anterior extremity 

 square, truncate, with the edges thin and prehensile ; the animal 

 attaches itself by this part, almost like a leech with its sucker, and 

 thus drags its body : posterior extremity broadly rounded. Above, 

 faintly coloured brownish purple in strise, with a colourless space 

 over the alimentary orifice. Length j^ths ; breadth yf (jths of an inch. 

 Hab. Congregated in numbers under stones, in brackish water ; 

 Chonos Archipelago (west coast of S. America) (December). 



The arrangement of the main branches of the intestinal cavity 

 is the same as in the terrestrial Planarice, with the exception of 

 the two posterior branches being united near the extremity of the 

 body into a ring, which structure I have not met with described 

 in any other species. Hence this species probably ought to form 

 the type of a new subgenus. 1 may here mention that I found 

 amongst these islands an elongated marine species (with a very 

 distinctly formed head placed on a narrow neck) which had the 

 power of crawling either backwards or forwards, — a power I have 

 never seen in any other species. 



4. Planmia (?) incisa. 



Plate V. fig. 3. Under-surface magnified. 



Body oval, very much depressed, highly contractile; margin sinuous, 

 anteriorly deeply indented, posteriorly less so. Ocelli very numerous 

 and crowded together in several rows on the indented anterior (as 

 is known by its progression) margin. Along the centre of the body 

 an intestinal vessel extends, and in the middle of this (B) there is a 

 well-closed orifice, through which the animal can protrude a thin, 

 much-folded, sinuated mouth-sucker ; this when fully expanded is 

 quite as wide as the body. Posteriorly, on each side of the central 

 vessel, there is a mass, apparently of immature ova. Near the pos- 

 terior extremity there is a second subterminal orifice (D), through 

 which, when the animal was placed in spirits, a little globular mass 

 was protruded, like a small, much-contracted mouth-sucker. Near 

 to the anterior extremity there are two slightly retractile paps with 

 orifices, of which the anterior one is the largest. From this point 

 diverging rays (intestinal cavities }) are sent off, which reach nearly 



