112 Papers from the Marine Biological Laboratory at Tortugas. 



branches. The other parts are perfectly differentiated. If the hypothesis 

 of the migration of the mesenchyme cells from the suppressed to the other 

 larva is correct, one would expect in this fusion correspondingly smaller 

 compensatory growth to have taken place; this is found to be the case, 

 namely additional processes on the ventral body rod shown at h and an 

 elongated dorso-ventral connective. 



A rather interesting instance of form equilibrium is seen in the A pluteus, 

 where the left dorsal arm is absent and the right has formed an accessory 

 arm rod and dorso-ventral connective. This fusion affords particularly 

 strong evidence that the skeletons of the two plutei do not fuse in the 

 ordinary acceptance of that term, for the overlapping of the parts is here 

 very considerable. 



In figure 9, the inequality of the two fused plutei is very great. The 

 A pluteus is perfect in every detail, the B is very incomplete. In the first 

 place, there is but one in place of four arms, and the gut is fused to the 

 foregut of the A pluteus. The skeleton consists of two ventral body rods, 

 only one of which possesses an aboral branch ; two dorso-ventral connectives, 

 and one large and one very small ventral arm rod. There is no trace of the 

 dorsal arm or body rods. As in previous instances, such suppression of the 

 skeleton is associated with compensatory growths, which, in this instance, is 

 confined to the supernumerary ventral body rod and to a marked thickening 

 of the other ventral body rod of the B pluteus. 



The fusion shown in figures lOA (somewhat enlarged) and in lOB is 

 particularly interesting. It resembles figure 9 in so far as the A pluteus 

 is complete and true to type, while the B pluteus is very incomplete. The 

 dominant pluteus has all the skeletal parts, the other (B) has but one ventral 

 arm and an atypic archenteron. The skeleton of this pluteus consists of 

 only three out of at least twelve parts, namely one ventral body rod, one 

 ventral arm rod, and one dorso-ventral connective. Over three-quarters 

 of the skeleton has not been differentiated. 



The compensatory growth of the skeletal structures is to some degree 

 commensurate with the degree of incompleteness or suppression. Close 

 examination of figure 10 will show that the right dorso-ventral connective 

 is not the normal single rod, but is much branched, and that from it a very 

 long bar extends across the body of A into the body of B ; it reaches almost 

 as far as the ventral body rod of B, and might easily be mistaken for its 

 symmetrical mate. Figure iob affords another view of this fused larva and 

 shows in a more striking manner some of the facts just referred to. In 

 passing, it might be noticed that the three short rods derived from the 

 dorso-ventral connectives of B are probably other supernumerary or com- 

 pensatory outgrowths of the skeleton. 



The extension of the large supernumerary (s) rod across one pluteus 

 and into the other might be accounted for in one of two ways. Either it was 

 formed originally as a ventral body rod in the B pluteus and secondarily 

 united with the skeleton of the A larva at its dorso-ventral connective, or 



