516 Comparative Animal Physiology 



Branchiomma vesiculostim, Cyathura carinata and Limnophila fusci-pennis: 

 for further details see von Buddenbrock^^). 



Negative geotaxis occurs less frequently among invertebrates, and is 

 illustrated by the pulmonate snails Umax and Helix. When these animals 

 are placed on an inclined plane in water, they ascend by the steepest 

 path,^"'-- "^ regardless of the slope of the inclined plane. These animals 

 have statocysts and, when they are supported by the water, proprioceptive 

 and other kinds of stimulation are minimal. When the snails are placed 

 on an inclined plane in air they choose a direction of ascent which deviates 

 from the steepest path by an angle which is related to the angle of inclina- 

 tion of the plane. Similar responses are obtained from a great variety of 

 animals which exhibit negative geotaxis. The simple and direct control of 

 orientation to gravity bv statocysts is certainly not indicated here. Rather, 

 in these animals, a given attitude is probably the result of a complex series 

 of reactions including several sense modalities. 



The numerous experiments of Crozier and his co-workers^** strikingly 

 illustrate the complexity of the situation. These investigators have tested 

 the responses of numerous animals on the inclined plane (Fig. 185). Their 

 results indicate that for many animals the angle of orientation up the plane 

 varies approximately as the common logarithm of the active component of 

 gravity, g sin a. Experiments with the beetle, Tetraopes tetraophthalmos,-^ 

 proved, interestingly enough, that addition of weight (i.e., wax) to the 

 abdomen resulted in an increase of Oj (the angle of orientation), whereas 

 the addition of weight to the anterior end or the removal of the abdomen 

 resulted in a decrease of 6 . Experiments with male fiddler crabs, Uca,^^ 

 which possess one large claw and one small claw, indicate that angle 9 is 

 greater when the large claw is on the downward side of the animal. These 

 and other experiments have led Crozier and his school to the conclusion that 

 orientation to gravity depends on distribution of the pull of the organ- 

 ism's mass on the supporting musculature; i.e., that the orientation is initi- 

 ated primarily by proprioceptive sense organs. That this may not be entirely 

 the situation is illustrated by experiments with ticks.^-^ Unfed ticks will 

 crawl freely in all directions on inclined or vertical planes; ticks gorged 

 with blood, however, always crawl upwards and travel more steeply, the 

 greater the angle of inclination of the plane. Similar results are obtained 

 when inert weights are attached to the abdomen of the tick. Thus, the 

 mechanical alignment of an organism on an inclined plane, due to a pos- 

 teriorly located center of gravitv, must be considered. The interpretations 

 of Hunter"'' suggest additional complications. A young rat on an inclined 

 plane has its center of gravitv above the plane, i.e., there is a tendency 

 for the animal to roll. This tendency to roll is greatest when the angle 9 

 is low, or when the angle of inclination, a, is increased. Thus, an increase 

 of a may be compensated by an increase of 9 . The orientation ol a young 

 rat and of other animals may thus be dependent on a series of postural re- 

 flexes invoh'ing statocysts, proprioceptors, and perhaps cutaneous sense 

 organs. 



The Vertebrate Labyrinth and Its Functions. The membranous labyrinth 

 of the inner ear of vertebrates is divided into two parts (see Figure 161, 

 Chapter 13): (1) the lower portion, including the sacculus and lagena or 



