266 BULLETIN OF THE 



tive larva. On the right-hand side of the cavity of the primitive 

 hydrophylUum is a cluster of cells of red color, which is the beginning 

 of a second similar serrated hydrophyllium. This latter cluster, how- 

 ever, has not raised itself any considerable amount above the surface of 

 the yolk. The longest diameter of the primitive hydrophyllium in 

 Fig. 14 is .75 mm.; the thickness at the distal side, .17 mm. The 

 length of the primitive cavity is .25 mm. ; its breadth is .10 mm. The 

 float is almost .10 mm. in diameter. It will be seen from these meas- 

 urements that the scale has now reached a very great size as compared 

 with its dimensions in earlier larvae, it has now the maximum size 

 to which it ever attains. 



Fate of the Primitive Hydrophyllium. 



It is known that this primitive hydrophyllium is a temporary or 

 embryonic structure ; but its fiite, whether it is simply thrown off or 

 absorbed, is not at present definitely made out. Both Hfeckel and 

 Metschnikoff have pointed out that it is a provisional structui*e, but 

 neither has traced it far enough in the last phases of its history to sat- 

 isfactorily show whether it is simply discarded, absorbed, or passes with 

 external changes of outline into some other structure. The most deti- 

 nite statement which we have is as follows. Metschnikoff says, " das 

 erstgebildete kappenformige Deckstiick abgeworfen wird" in the genus 

 Agalma. 



The primitive hydrophyllium of Agahna elegans suffers many modifi- 

 cations in external form in some of the older stages ; but whether these 

 modifications were abnormal, resulting from the fact that the animal is 

 in confinement, was not determined. It seems to me more natural to 

 suppose, that, instead of being thrown oft' in the subsequent stages, the 

 primitive covering-scale passes by a few modifications in its external 

 contour into some other organ, probably a diftcrcntly formed covering- 

 scale. 



Fig. 15 represents the larva of Agalma on August 10, four days after 

 the capture of the parent. This larva was picked out of the water, 

 in which it was freely swimming below the surface. The figure repre- 

 sents the larva as seen from that pole which is opposite the germina- 

 tive pole, so that the various organs which have appeared near that 

 region are seen through the yolk contents. This position, assumed 

 while <-he egg '" floating, is that which is best adapted to exhibit the 



