i62 THE BIOLOGY OF THE SEA-SHORE 



headings. Like snails, Turbellarian worms lay their own 

 track in the shape of a glandular secretion which bathes the 

 body and enables the cilia to work equally well on different 

 substrata. The common Planarian Dendrocoelum lactea, 

 whose usual method of progress is like that of Leptoplana, 

 is said to perform, if alarmed, a series of rapid " looping " 

 movements by affixing a sucker, situated on the under surface 

 of the head, to the substratum and pulling up the posterior 

 end close behind this (Gamble, loc. cit.). 



A considerable amount of research has been directed 

 towards the mechanism of locomotion in Gasteropods, which 

 are essentially gliding forms. The organ of locomotion is, 

 of course, the muscular " foot " which, morphologically, is 

 a thickening of a portion of the ventral surface of the body 

 wall, and whose under surface in the univalve molluscs is 

 a flat sole. Modifications of structure occur in relation to 

 the nature of the normal habitat, and in many forms there is 

 a certain differentiation into regions called propodium, 

 mesopodium, and metapodium respectively. The anterior 

 portion or propodium is most strongly developed in genera 

 which crawl about in wet sand, e'.g. Natica, Sigaretus, Oliva, 

 Harpa, Scaphander, where *' it seems to serve as a sort of 

 fender or snow-plough, to push the sand away on both sides 

 of the path the animal is traversing " (Cooke, 1895). Accord- 

 ing to Woodward (19 13), the expansion of the foot in the 

 Naticidse is assisted by the presence of aquiferous spaces, 

 which are completely separated from the circulatory 

 apparatus. On the other hand, in forms such as Buccinum 

 undatum the lower margins of the foot are greatly extended, 

 which allows the animal to crawl on the surface of the sand 

 without sinking in (Woodward, loc. cit.). 



The mechanism of locomotion may be studied by allow- 

 ing the mollusc to creep over a piece of plate glass. When 

 a snail is observed in this way the optical effect, according 

 to Woodward, " suggests a rapidly flowing stream pro- 

 ceeding from the tail to the head, and this is due to the 

 successive raising, moving forward and replanting down of 

 succeeding portions of the under surface of the foot, and is 



