io8 Papers from the Marine Biological Laboratory at Tortugas. 



FUSED PLUTEI. 



Agglutinated or "true twin" larvae are of no special significance for my 

 present purpose, for each is perfectly developed and no reciprocal influence 

 occurred. Figures i to 3 correctly represent any such agglutinated larva. 

 It is only in fused larvae, i. e., larvae with a common body cavity and a com- 

 mon and continuous ectodermal wall, that reciprocal changes are observed. 



D.h.J 



D.b.r. 



In the following drawings, for the sake of simplicity, I have not always 

 indicated the whole skeleton when such omitted portions are normal and 

 when the symmetrical right or left side is fully represented. Figure 5 is 

 a partial fusion of two larvae with a common body cavity and a continuous 

 body wall. Each larva is completely differentiated and normal. The 

 archentera are complete and independent, though they appear to be super- 

 imposed in this particular view of the fusion. The skeletons are typical 

 and the parts are complete and characteristic of the normal single larva. 

 On closer examination it is found that certain parts have not developed or 

 have not reached their full size, such as the left dorsal rods of the right or 

 A pluteus, and the left dorsal arm of the left or B pluteus. Associated 

 with this suppression or retardation of the dorsal rods there is a compen- 

 satory growth of the left dorsal arm of the B pluteus. This supernumerary 

 arm bifurcates, one branch extending into the right pluteus, the other branch 

 into the left pluteus. And though the skeletal rods of the two larvae are 

 in close proximity and overlap in several places, there is no trace of a fusion 

 of the rods. This suppression of the dorsal rods and the associated compen- 



