REPORT OF COMMISSIONERS OF INLAND FISHERIES. 203 



It will be observed, further, that, when the thoracic appendages 

 take the "anterior" position, the direction of the strokes of the 

 exopodites becomes somewhat forward, as well as downward; and' 

 the resulting motion of the larvae becomes upward and backward. 

 When, on the other hand, the thoracic appendages assume the 

 "posterior" position, the stroke of the exopodites becomes backward 

 and downward; and the resulting motion of the larvse becomes 

 forward and upward. During a great part of the time the upward 

 movement of the larvae, as the result of the outward and dowmward 

 stroke of the exopodites, does little more than compensate for the 

 natural tendency to sink toward the bottom. For this reason, the 

 progression of the larvae may be often directly forward or directly 

 backward, with but slight deviation from the horizontal plane; while 

 at other times, when the stroke of the exopodites is directly outward 

 and downward (exclusive of either the "forward" or "backward" 

 factor), the larvae may mount to the surface in nearly vertical lines. 

 Obviously enough, however, there may be many different directions 

 of swimming, as determined by the degree of extension or contraction 

 of the thoracic appendages. 



It thus becomes evident that the progression of the larvae, back- 

 ward or forward, upward or dow^iward, is largely determined by the 

 position (the state of extension or contraction) of the thoracic 

 appendages. In other words, if , for the greater part of the time, 

 these appendages are in the "anterior'' position, the reaction of the 

 larva is positive; but if the thoracic appendages are more frequently 

 in the "posterior" position, then the consequent reaction of the larva 

 is negative. Naturally, the next important question which arises 

 is: What conditions determine the "anterior" or the "posterior" 

 positions of these thoracic appendages? It can not be questioned 

 that these changes are directly due to certain variations in the 

 intensity of the illumination, and are also modified by the "physio- 

 logical state" of the larvae themselves; that, furthermore, the state 

 of extension or contraction of the thoracic appendages (and conse- 

 cjuently the direction of the stroke of the exopodites,) is regulated 



