Experimentally Fused Larvce of Echinoderms , etc. 



Ill 



and their shape and size and relative positions are essentially normal. The 

 skeleton of the B pluteus is far from complete. It contains the two ventral 

 body rods and two dorso-ventral connectives which are quite normal, but 

 the two dorsal body rods, one of the ventral arm rods, and one dorsal arm 

 rod are very much reduced in size, and there is no trace at all of the aboral 

 branches. The compensatory growths of the skeletons in this fusion 

 are very striking indeed. They are altogether limited to the A pluteus 

 and confined to processes on the right ventral body rod at H^ and on the 

 left ventral body rod at W^, where the outgrowth is much branched and 

 extends into the B pluteus. The formation of these hypertrophied and 

 supernumerary bars is explicable on the assumption that when the mes- 



d.v.c. 



'-v.b.r. 



Fig. 8. 



Fig. 9. 



enchyme cells were prevented from developing within its own larva they 

 migrated into the adjoining or A larva and there formed the additional 

 skeleton at or near the center of active skeleton building, namely, the region 

 of the union of ventral rods and dorso-ventral connectives. 



Figure 7B is a dorsal view of the same pair of fused plutei, which shows 

 more clearly some of the points referred to. 



Figure 8 represents two plutei much more completely fused into one 

 body. The dominant or A pluteus is typical and complete; the suppressed 

 or B pluteus is smaller and incomplete, and this inequality is shown par- 

 ticularly well in the skeletons. It will be observed that the B pluteus is 

 deficient in these skeletal parts that were missing or abbreviated in the 

 previous examples, namely, the dorsal arm rods and the aboral ventral 



