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 resumé 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 extrément fines de tous les petits faisceaux longitudinaux 
qui composent cet empatement.’ ” 
“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 Flussigkeit 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 Lymnzus 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 
