Kew : Slime -Threads of Planar inn -Worms. 313 



withdrew ; or, as sometimes happened, it broke through the 

 surface-film and fell into the water. * 



Turning- now to the water-planarians, we find that some, like 

 the land-kinds, are known to draw out threads. 



The freshwater Tricladida, the common planarians of the 

 ponds and streams, are somewhat flatter and less slender than 

 land-planarians, and are of but moderate size, hardly ever 

 exceeding an inch in length, and usually much smaller ; they 

 progress with an even, gliding motion, and, like land-planarians, 

 etc., they leave a slime-trail in their path; f like molluscs, they 

 creep, not only on solid surfaces, but also on the under-surface 

 of the water ; and this they do, as shown by Willem { and by 

 Lehnert, by means of the floating slime-trail which the animal 

 deposits as it proceeds. 



Of these planarians we have notes for two genera : Planaria 

 and Polycelis ; and we find that the thread, which they make 

 during descent through the water, may start, either from a 

 slime-trail on a solid body, or from a floating trail at the water's 

 surface. 



The thread was first noticed, in 1814, by Sir J. G. Dalyell, 

 who kept hundreds of individuals of planarians of this kind in 

 captivity in glass vessels. This naturalist writes of Polycelis 

 cornuta % that when dropping from the surface of the water to 

 the bottom of the vessel (a habit common to other species) ' it 

 seems to exercise a faculty belonging to the caterpillar, of 

 spinning a silken thread, visible only when so much aggregated 

 as to interrupt the rays of light.' The purpose of the thread, 

 the author supposes, is to check the rapidity of the descent ; 

 and a complete view of its effect may be obtained, he adds, by 

 including a plant of Veronica, crowded with planarians, within 

 a tall glass jar. The numerous descents from the upper leaves, 

 in this case, 'quickly form a perceptible column, owing to the 

 infinity of glutinous or silken lines. '|| In a later work, under 

 ' Plana ria arethusa ,' the author writes : — 



This animal, in common with its kind, is protected from abrasion by 

 a superficial glutinous secretion investing- the bod}', whereby it may, 



* Lehnert, I.e. 



1 J. R. Johnson, Philosophical Transactions of the Royal Society, [822, 

 p. 439 ; Lehnert, I.e. 



X Willem, Bulletins de l'Academie Royale des Sciences, des Lettres et 

 des Beaux-Arts de Belgique (3), XV. (1888), p. 429. 



§ Planaria felina. 

 Dalyell, Observations on Planaria;, 1814, pp. 4^-7. 



iqoo October i. 



