Experimentally Fused Larvce of Echitioderms, etc. 



107 



this process. This paper will deal with the consideration of the changes in 

 the skeletal structures in fusing larvae. 



SINGLE PLUTEUS LARVA. 



Except where specifically mentioned to the contrary the drawings were 

 made with the camera lucida and drawn to the same scale. Figure i is a 

 lateral view of a single larva fully devloped. The figure represents the 



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Fig. I. 



larva somewhat foreshortened, but with all of the parts, particularly the 

 skeletal structures, outlined. Within the body of the larva are two ven- 

 tral body rods, one on each side of the body. From these rods two long 

 bars extend into the ventral arms. At right angles to these, and arising from 

 the union of the ventral body rods and the ventral arm rods, are two ventro- 

 dorsal connectives, which in turn bifurcate, one branch extending into the 

 dorsal arm and the other aborally into the body to form the dorsal body rod. 

 From the aboral end of the ventral body rods are two usually bifurcated 

 aboral branches. Finally, there are two ventral branches between the 

 ventral arms, but these are not shown in the figure. These bars are easily 

 recognized from one another in both the single and fused larvae, and they 

 afford a definite and measurable index of the changes during the fusion of 

 the larvae. 



Figure 2 is another single larva drawn under greater magnification, 

 It shows, from a foreshortened ventral view, the same characteristic parts 

 of the skeleton, and also illustrates the small knobs and processes on the 

 ventral rods, which are sometimes not reproduced in the drawings. Figures 

 3 and 4 are enlarged to the same scale as figure 2 and represent the ventral 

 and lateral views of the skeleton. Figure 4 affords a particularly good 

 example of the spinous ventral body rods. 



