INLAND FISHERIES. (53 



the body and poiut backward, but in each succeeding' stag-e they 

 tend to poiut further forward, so that in the fourth sta<i-e, when 

 the animal swims, the big claws meet in front of the head. The 

 surest and quickest means of disting-uishing them is, however, l)y 

 the appeudag-es on the underside of the abdomen (tail). In the 

 first stag-e there are none. In the second stage the several pairs 

 of swimmerets are developed (Fig-. 2). In the third stage the ap- 

 pendages on the segment at the tail end appear (Fig. 3). The 

 fourth stage is so diflerent from the previous ones that no diffi- 

 culty is ever experienced in detecting it. 



2. What is the duration of the first three stages f — The average 

 period between hatching and reaching the fourth stage for the en- 

 tire eleven experiments at Wickford was a little over twelve days. 

 In each experiment the average duration of the first tlu-ee stages, 

 meaning the interval between the time of hatching and the day 

 upon which the largest number entered the fourth stage, is given 

 in the table on page 78, and varies from nine to sixteen days. 

 From the first experiments, begun June 1, to the later ones there 

 is a variation in the length of the period among diiferent indi- 

 viduals, amounting to three or four days at the least, and in 

 some instances even to six or seven days. 



In experiments conducted at Wood's Hole the time required 

 for these moults was considerably greater; of the first lot, 

 hatched May 23d, the first stage was reached by a few only on 

 June 12th, after an interval of twenty days. Indeed, on the twelfth 

 day (the average time of reaching the fourth stage at Wickford) 

 none had reached even the third stage at Wood's Hole. The ex- 

 planation of the variations in the length of time required for the 

 first three stages probably lies in the differences in temperature 

 of the water — the colder the water, the slower the deveh)pment. 

 An examination of the tables referred to bears out this con- 

 jecture, thus : 



