JOHN W. SCOTT. 



on the third, fourth and fifth segments, but the inner openings 

 are all anterior to the diaphragm. The inner openings of the 

 post-diaphragmatic nephridia are fimbriated membranes, consist- 

 ing of dorsal and ventral portions in close apposition, covered by 

 cilia ; each of these openings leads into a large membranous sac 

 that is well supplied with blood capillaries and is also ciliated. 

 These nephridia serve as gonaducts, and probably serve also as 

 excretory organs. When egg-laying was first observed, I thought 

 the collapse of the egg in the polar diameter might have a direct 

 relation to the separating process, but there is nothing in the 

 structure of the nephridia to indicate that the eggs undergo a 

 sifting process. Besides the coelomic corpuscles do not escape 

 and they are smaller than either ripe or unripe eggs. 



Somewhat similar phenomena have been observed by Gerould 

 ('06) in Pliascolosoma. " A few hours before egg-laying occurs, 

 the nephridia become distended with a transparent fluid." " Ova 

 that are ready for maturation, having the spindle of the first polar 

 body in metaphase, are swept from the ccelom into the nephridia 

 by the action of cilia which give rise to strong currents within 

 the nephridium, setting from the nephrostome backward towards 

 its posterior extremity. This is a most interesting process in 

 that both the immature ovocytes, which are present in great 

 numbers in the ccelomic fluid, and the coelomic corpuscles are 

 excluded from the nephridium, while the fully grown ovocytes 

 are collected there in great numbers." Gerould presents a 

 tentative explanation somewhat as follows. The transparent 

 fluid, he thinks, is sea-water, taken in through the nephridi- 

 opore, and he believes that ova in the early stages of maturation 

 probably absorb water while within the nephridium. " If eggs 

 in the earliest stages of maturation show a tendency within the 

 nephridium to absorb sea-water, may it not be assumed that ova 

 at that stage are positively hydrotropic ? On this supposition 

 we may explain why such eggs are caught up from the ccelomic 

 currents into the nephrostomal region, and thence carried into 

 the nephridium." This assumption, however, is not to be seri- 

 onsly regarded and is, I believe, incorrect ; at least it is incorrect 

 in the case of Amphitrite. 



It is rather an easy matter to separate the ripe eggs from the 



