130 Papers from the Department oj 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 

 cases 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 from the FIO. i. Laganum piuteus, 29 hours, viewed 



Center Of the dorsal margin Of the ventrally and somewhat anteriorly. 



..... , op., opening into ammo tic cavity. X 130. 



anterior surface, while in the lat- 

 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 piuteus 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. 



