with an Account of their Habits. 243 



tion), except in the entire absence of ramifications on the internal 

 sides of the two posterior prolongations of the main digestive ca- 

 vity. There is generally a colourless space round the alimentary 

 and genital orifices. The mouth-sucker is bell-shaped, with a 

 very short oesophagus : when contracted it forms either a globu- 

 lar or star-shaped hard ball : I never saw it voluntarily protruded, 

 but have no doubt that it can be, for immersion into very weak 

 spirits of wine or salt water caused its exsertion, and on being 

 touched it was immediately retracted. This mouth-sucker is 

 highly contractile, and retains its irritability long after the death 

 and even dissolution of the rest of the body : the external orifice, 

 through which it is protruded, consists of a transverse slit. The 

 genital orifice, also, consists of a transverse slit ; in the aquatic 

 species it is generally, if not always, circular. In my notes on 

 several of the species, I find it stated that the under surface or 

 foot is thickly studded with very minute, angular, opake, white 

 specks : may not these serve for the necessarily copious secretion 

 of slime ? These animals, when placed on a slip of glass, fre- 

 quently propelled a globule of air, between their foot and the 

 glass, from their anterior extremity towards their tail ; and as the 

 air came in contact with successive parts of the foot, a violent 

 corpuscular movement (curiously resembling microscopical eels 

 disturbed by a stick, and struggling in mud) was produced in the 

 slimy surface. I could never perceive it in any part of the foot, 

 except when in contact with air ; but it was evident, though less 

 energetic, on parts of the back, and at the extreme anterior extre- 

 mity of the body. I presume that the appearance is due to vi- 

 bratile cilia ; and it is worthy of remark, that M. Duges* suspects 

 that the foot, in the freshwater species, is the chief seat of this 

 respiratory action, from having observed that they frequently arch 

 their bodies, so as to allow fresh water to circulate under it. The 

 position of the black eye-spots varies in the different species : it 

 is remarkable that, in the P. elongata from Tres Montes, I could 

 perceive no trace of these ocelli, although this is the largest spe- 

 cies. According to Prof. Ehrenberg's arrangement, depending 

 on the presence and number of the ocelli, this species would rank 

 in his genus Typhoplana ; but from the variability in number and 

 position of these imperfect organs of vision, I should doubt whe- 

 ther they ought to afford generic characters. In the P. pallida 

 I examined the ocelli with a strong lens, and found that they 

 were not truly circular ; the black part lies within a transparent 

 envelope ; in this species they are seated on the upper margin of the 

 body, in groups of two and three, exactly over the extreme lateral 

 subdivisions of the intestinal vessel. I was not able to see ova 



* Annales des Sciences Naturelles, October 1828, p. 28. 



