316 



Kew: Slime -Threads of Planar ian -Worms. 



the vessel, he adds, no nets are spun.* In most planarians, the 

 slime — often charged with minute offensive rods — plays an 

 important part in the overpowering- of prey; but it is improbable, 

 as it seems to the writer, that nets of slime are ever specially 

 spread for this purpose ; the nets in which the larvae of Noto- 

 necla were caught, one can well imagine, may have consisted 

 merely of threads or trails of mucus deposited by the Mesostomas 

 during their wanderings through the water and along its surface. 

 As regards the source of the slime, Schneider describes in detail 

 certain glands (Spinndriisen), distributed over the ventral sur- 

 face, chiefly in the median line. 



In reviewing the facts now detailed, the chief point which 

 presses itself upon our attention is that mentioned at the 

 outset — the similarity of the observations with those recorded 

 for the very different animals, Mollusca-Gastropoda. The 

 behaviour of land-planarians as above noted, down to some 

 of the small details, is surprisingly similar to that of land-slugs ; 

 and the conclusions of Lehnert on the production and use of the 

 thread of Bipalium and Geodesmus are almost identical with 

 those at which the writer has arrived with regard to the thread 

 of Limax and Arion. it is in the length to which the thread 

 may extend that the chief difference occurs, for while the longest 

 thread recorded for a land-planarian was little more than 

 three times the length of the animal, a land-slug (which, how- 

 ever, is a less slender creature) has been observed by the writer 

 to make a thread more than 70 times its own length, and still 

 longer threads have been recorded by authors ; further, no land- 

 planarian appears to have been seen to ascend the suspensory 

 thread, a thing which slugs do with some facility. This latter 

 break in the analogy, however, is filled by the water-planarian, 

 Polycelis, which, like a water-snail, can ascend its somewhat 

 lengthy thread. In other respects, moreover, the water- 

 planarian's use of a thread is very similar to that of some 

 aquatic molluscs ; both kinds of animals, it may be noted, 

 descend, not only from aquatic plants, but also from the surface 

 of the water. The bridge-threads referred to by Lehnert are of 

 the nature of similar threads made by slugs and other molluscs. 



The planarian's thread used for descent is probably formed 

 chiefly when the animal has to escape from dangerous surround- 

 ings ; and this is true, presumably, both for . water and land 

 kinds. With regard to land-slugs, the writer has concluded 



* Schneider, I.e. 



Naturalist 



