314 Kew: Slime -Threads of Planar iim -Worms. 



perhaps, find the means of its suspension in the water, as if by a thread, 

 which is frequently practised. If truly so, we need not seek for any 

 secretory organ near the posterior extremity, as appearances might lead 

 the observer to conjecture. Many having congregated on a plant hung in 

 the vessel containing them, some detached themselves successively in order 

 to descend, but several individuals, at once, taking advantage of a thread 

 belonging to their neighbours, which was discernible by a magnifier, broke 

 it from their accumulating numbers, and the load precipitated amidst the 

 fluid, dispersed below. The glutinous matter may be drawn off the body 

 of the animal on quitting its hold or adhesion to any surfaces.* 



Further observations have been made by Lehnert, who refers 

 in this connection to Planaria lacteal and PI. polychroa, but 

 principally to the Polycelis lentils Iijima : an animal, about 

 Y2, in. long", apparently allied to our common Polycelis nigra. 

 The character of the spinning-, it is said, is the same as that of 

 land-planarians ; but, owing to the water-plana'rian's smaller 

 specific gravity, the thread can be continued to a greater length 

 — many times the length of the animal. The Polycelis, more- 

 over, Lehnert says, can climb up its suspensory thread! : an 

 observation which appears to be unique as far as the planarians 

 are concerned. 



For Polycladida we have no observations, and may thus pass 

 to the remaining tribe — Rhabdoccelida — small, often minute 

 creatures, progressing by creeping both on solid bodies and at 

 the water-surface ; and capable also of swimming - . 



Our notes, for Rhabdoccelida, relate to two very dissimilar 

 genera, Monotus and Mesostoma. 



Dalyell remarks of the minute littoral Monotus lineatus § 

 that it is ' apparently sustained by an invisible thread when 

 falling through the water ' || ; and Jensen and von Graff have 

 referred to the thread of the allied Monotus fuscus.** The latter 

 animal, it is said, congregates in swarms, in captivity, at the 

 surface of the water, and when the vessel is shaken the creatures 



*Dalyell, The Powers of the Creator displayed in the Creation, II. (1853), 

 p. 113. ' Planaria arefhusa,' as Mr. F. W. Gamble obligingly informs the 

 writer, is probably the form now known as Planaria alpina. Dalyell further 

 notes (torn, cit., p. 116) that his marine ' Planaria variegata,' when dropping 

 through the water, seems to be retarded by an invisible thread ; but this 

 animal, as suggested by Johnston (Catalogue of Non-parasitical Worms in 

 the British Museum, 1865, p. 1 2), is probably a mollusc. 



\De?idroccelum lactenm. 



JLehnert, I.e. 



§ Planaria Jlustrce. 



|| Dalyell, 1853, torn, cit., p. 118. 



** Monocelis assimilis (in Jensen). 



Naturalist, 



