130 Papers from the Department of Marine Biology. 
enlarged with camera drawings magnifying to twice the number of 
diameters. Figures 2 to 15 have been taken from the 55-hour speci- 
mens, all others from the 29. 
INTERNAL STRUCTURE. 
The body of the 29-hour plutei of Laganum is ovoid in shape, in some 
eases almost spherical, with a flattened or slightly concave anterior 
surface (fig. 1). Two larval arms grow out from a position somewhat 
dorsal to the center of the antero-lateral regions. In the 55-hour 
specimens, the body has elongated slightly and the posterior end has 
become somewhat pointed. In the earlier stage the arms extend out- 
ward at an angle of about 60° from the body, while in the older form 
they are more slender and reach upward nearly parallel to each other. 
An opening to the exterior lies in the center or slightly to the right 
of the center of the flattened anterior surface. It may be seen readily 
in the whole mount, but no other 
opening can be observed. The 
animal itself is exceedingly opaque, 
so that no study of the internal 
organs could be made except in speci- 
mens especially fixed and bleached 
for observance of the skeletal struc- 
tures. Unfortunately, in this de- 
tailed treatment the tissues were 
so largely destroyed that study of 
them was impossible. 
Among the specimens studied, 
about fifty in all, I have found three 
which show interesting variation in 
structure—two with 3 larval arms, 
and one with 5. In the former case 
the third larval arm arises fromthe Fi. 1.—Laganum pluteus, 29 hours, viewed 
center of the dorsal margin of the ventrally and somewhat anteriorly. 
anterior surface, while in the lat- tek, taBon eee 
ter instance there are, in addition to this one, 2 on each antero-lateral 
margin, one immediately dorsal to the other. All the arms are of 
about the same size, and in other respects the pluteus seems to be nor- 
mal, but since these three specimens were found among those preserved 
especially for the skeletal structure, no further study of the internal 
anatomy could be made. The possibility exists, of course, that they 
represent the normal development of a later stage, but this scarcely 
seems probable in view of the complex condition already attained by 
the remaining portion of the larval skeleton and the advanced stage of 
development of the echinoid rudiment and permanent skeletal struc- 
tures. 
ation, 
are 
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