34 THE CHICAGO ACADEMY OF SCIENCES. 



Two other means of locomotion may be observed. One is that 

 of rising suddenly from the bottom to the surface of the water, and 

 the other is that of dropping suddenly from the surface to the bot- 

 tom. These are both caused by the ability of the animal to increase 

 or decrease its specific gravity by the presence or absence of air in or 

 on its body or shell. 



MECHANISM OF LOCOMOTION. The mechanism by means of which 

 locomotion in snails is performed has been the subject of dispute and 

 the question is far from being answered in a satisfactory manner. 

 Walter (1. c., p. 8) has prepared a resume of the subject, which is 

 repeated below : 



"Blainville, as quoted by Moquin-Tandon, rather indefinitely sug- 

 gested that locomotion was caused by an undulatory muscular motion 

 resembling somewhat that of a snake, but quite dissimilar in origin. 



" 'C'est plutot un glissement du disque abdominal produit par des 

 ondulations extrement fines de tous les petits faisceaux longitudinaux 

 qui composent cet empatement.' r ' 



"Quatrefages in 1850 assigned the cause to the motion of the cilia 

 clothing the foot rather than to any particular motion of the foot itself. 

 Simroth advanced the hypothesis of 'extensile muscles' which continu- 

 ously lengthen anteriorly' and shorten posteriorly; while Maria Grafin 

 von Linden has in mind a wave-like motion ('Wellenspiel') when she 

 says : 'Die kleinen wellenformigen Bewegungen auf dassen Sohle 

 geniigen um eine Ortsveranderung hervorzubringen.' Sochaczewer 

 considered the locomotive force to be the blood flow in the cavernous 

 tissue of the foot. To quote his words: 'Die Fasern selbst werden 

 durch die einstromende Fliissigkeit gedehnt und wirken erst nach re- 

 flektorischen Anreiz treibend auf die Maschen anschwellende Blut- 

 menge.' Car came to the conclusion through anatomical studies that 

 locomotion is due to a combination of contractions and relaxations of 

 longitudinal and dorso-ventral muscles, but Simroth, returning to the 

 controversy, pointed out that other forms than Lymnaeus which have 

 no dorso-ventral muscles at all, progress over the surface of the water 

 equally as well. 



"Jordan advances a theory based on the pressure of isolated bodies 

 of visceral fluid or blood which is somewhat similar to that of Sochac- 

 zewer, and finally Carlson finds that certain land pulmonates appear 

 to progress by waves of relaxation and contraction wherein the mus- 

 culature of the dorsal and lateral walls of the body cavity act in such 

 a way that the whole foot is not in intimate contact with the substra- 

 tum at the same time. The contraction of the longitudinal muscles 



