Experimentally Fused Larvce of Echinoderms, etc. 



115 



larva, and a very incomplete larva represented by a single broad arm, in 

 which there is a ventral arm rod, a ventral body rod, and a dorso- ventral 

 connective. In figure 13 there are two thin, straight bars one long, the 

 other very short, neither of which connects with either the B or the A 

 skeletons. If it be assumed that retrogressive changes have taken place 

 and that these have affected one or both ends of certain rods, we should 

 expect the connections between rods to disappear, the distal ends to be 

 shortened, and the bars themselves to be thinner than corresponding rods 

 of control larvae. All these results are found in figure 13 and the inde- 

 pendent bars probably represent the former dorsal and ventral arm rods. 



Fig. 12. 



Fig. 13. 



In figure 14 form equilibrium is more closely established. Larva B is 

 reduced to a small, very blunt protuberance on the left side of the figure. 

 It consists internally of a ventral body rod and its aboral branch, a very 

 much dwarfed ventral arm rod, a dorso-ventral connective, a dorsal body 

 rod, a supernumerary rod at S and a mere rudiment of the dorsal arm rod. 

 In other words only one half of one side of the skeleton has been formed in this 

 larva. 



The gradual decrease in size and contents of the B larva, seen in figures 

 II to 14 inclusive, reaches an extreme state in figure 15; and if it were not 

 for the intermediate stages already described, figure 15 might easily be 

 overlooked or mistaken for a single larva. 



In figure 15 the A larva contains all the parts in their proper relative 

 positions typical of the control larva. The gut is somewhat enlarged, 

 indicating the incomplete fusion of the two archentera. The skeleton is 

 single and perfect and to describe it is to describe the control larva. The B 



