202 REPORT OF COMMISSIONERS OF INLAND FISHERIES. 



with the mechanics of 'progressive orientation, and then turn with 

 better understanding to the mechanics of body orientation; for these 

 two are presumably dependent upon quite different circumstances. 



THE MECHANICS OF PROGRESSIVE ORIENTATION. , 



As we have already observed, the only means of locomotion pos- 

 sessed by the larvae of the first three stages are the exopodites of 

 the thoracic appendages, and the strong flexible abdomen with its 

 broad terminal fan. It is but seldom that the latter is used, however, 

 and never when it is a question of progressive orientation to light. 

 We are then confronted with the problem: How, with the action of 

 the thoracic exopodites alone, is the larval lobster able to execute 

 those movements which shall determine his motion either toward 

 the source of illumination or away from it? The superficial expla- 

 nation of this phenomenon is simple enough; and it is probably due 

 to its very simplicity that it has remained so long a time unde- 

 termined. 



If the larval lobsters in any of the first three stages be put in a 

 glass jar which is surrounded by black paper and placed in subdued 

 daylight, their swimming activities may be studied to the greatest 

 advantage. The short vibratory strokes of the exopodites may be 

 readily observed. At one time, certain individuals may be seen to 

 swim rapidly backward, and again forward, without any apparent 

 change in the position of the body or in the direction of the stroke 

 of the exopodites. If, however, the thoracic appendages themselves 

 be carefully watched, one can observe that, from time to time, these 

 limbs undergo either a forward shifting (extension) or a backward 

 shifting (contraction). These changes from the "anterior" position 

 to the "posterior" position may occur successively and at short 

 intervals; the "forward" or the "backward" position may persist 

 for some seconds, or again, there may be a successive alteration, 

 with periods of longer duration in either one or the other of these 

 positions. 



