THE LIFE-HISTORY OF HYDROBIUS FUSGIPES, L. 329 



Within the posterior abdominal segments are traces of the lateral tracheal trunks 

 which are just forming. By this time all trace of the abdominal appendages has 

 disappeared. 



On the tenth day the trophi and legs show segmentation and have lengthened out 

 considerably, while the segmentation of the body somites is also well advanced, proceed- 

 ing forwards from the posterior end of the abdomen. The lateral tracheal trunks are 

 now quite distinct in the posterior part of the abdomen as colourless tubes. 



The eleventh day seems to be the muscle-forming period, as at this stage muscle 

 fibres are visible in various parts of the body. The lateral tracheae which have not yet 

 developed through the whole length of the embryo show, posteriorly, rudimentary 

 branches, and in the median dorsal line faint traces of the heart are visible in the 

 posterior segments. 



The most marked change on the twelth day is the extension forward and the 

 pigmentation of the lateral tracheal trunks. These now reach into the head and show 

 indications of the main branches and of the future " setose gills." 



On the thirteenth day the tracheal system is almost completed, the dorsal network 

 having been formed, and the branches of the head can be seen tapering off to fine threads, 

 but, so far as I can make out, the "setose gills"* themselves do not appear until the 

 fourteenth day. There are, however, only small differences noticeable in the living egg 

 of the thirteenth and the fourteenth day. 



The fourteenth day is the last stage, the embryo now being complete within the 

 shell. One point to notice is that, almost from the commencement of development the 

 embryo has never completely occupied the shell, there having all through been a fair 

 amount of free space. On the fourteenth day, however, the embryo commences to swell 

 up two or three hours previous to its escape, so that at the time of the bursting of the 

 shell there is no visible unoccupied space within the shell. 



While watching this process of swelling up of the embryo I found in the head, 

 situated in the median line and immediately below the dorsal aorta, a pulsating 

 organ. It pulsates much more slowly than the heart, only commences to pulsate a few 

 hours before the embryo escapes, and does not, so far as I have observed, pulsate con- 

 tinuously from the time it first starts. It begins with a few beats, after which there is 

 a pause of, may be, many minutes, but as the time of emergence approaches the pauses 

 are fewer and the pulsations rather more rapid, being about 40 per minute, while the 

 the heart is beating at quite 100 per minute. 



This pulsating organ is driving fluid backwards, but it was not possible to see where 

 the fluid came from or was driven to. I discovered a ligament suspending the organ, 

 and as it passes on one side of the aorta, I expect that it is one of a pair. 



There are no blood corpuscles visible in the fluid pumped through this pulsating 

 organ. In conjunction with its pulsations certain movements of the gut are visible, and 

 these become more noticeable as the beats of the pulsating organ become more frequent. 



* Vide infra. 



