MUSEUM OF COMPARATIVE ZOOLOGY. ral 
resemble the nuclei of the epiblast as already described, are well 
marked on the ridges of the scale. 
I have been unable to identify a scale of this kind with any of those 
figured in the larval stages of Agalma as described by Metschnikoff. 
Heckel, however, figures a similar scale with divided tube in Crystallo- 
des, but from his descriptions it does not follow that he regards it as the 
modified primitive scale. In Physophora, however, we find an approxi- 
mation in shape to this scale in the primitive hydrophyllium, and more- 
over in this genus, as in mine, there is a smaller tube extending from 
the cavity of the scale to the surface, and ending in or near clusters 
of lasso-cells superficially placed. If the first-formed scales (primi- 
tive hydrophyllia) in both Physophora and Agalma are homologous, we 
may find the smaller bifurcations connecting the cavity of the scale in 
Agalma with its surface to be the same as the similar structures de- 
scribed by Heckel in the young Physophora, provided, of course, that 
the flat scale of Fig. 8 is the modified primitive covering-scale of Pl. III. 
fig. 14. The flat scale (fig. 8) is certainly different in the contour 
and course of the central tube from the serrated hydrophyllia, and no 
other structure is thought of to which to refer it except the primitive 
hydrophyllium, that large covering-scale whose origin dates back into 
the youngest stages of the larva. What has already been here written 
of the modifications in form which the first-formed covering-scales go 
through, does not of course show that in the end it may not be simply 
cast off. My studies throw no light on this point. If it is ultimately 
dropped it undergoes modifications in outline before the consummation 
of that event. 
CAMBRIDGE, July, 1885. 
