114 Papers from the Marine Biological Laboratory at Tortugas. 



-v.b.r. 

 d.b.r. 



archenteron, etc. The other or B pluteus is represented externally by a 

 single arm. This arm, however, contains a ventral and a dorsal arm rod 

 united by a dorso-ventral connective, from which a characteristic ventral and 

 a dorsal body rod extend into the common body cavity. The arm of B 



is really the right or left half of the 

 B larva and if there be any doubt 

 as to the dual character of figure 

 II, the diminutive archenteron in 

 the B larva should at once set it 

 aside. 



There are several instances of 

 compensatory growths in the 

 dominant larva. There is the 

 hypertrophied dorso-ventral con- 

 nective, and better still the dorsal 

 body rod which is not merely 

 considerably longer but extraordi- 

 narily thicker than in the con- 

 trols, and finally an elongated left 

 dorsal body rod. 



It should again be noted that 

 although there is considerable 

 overlapping of adjoining skeletons 

 there is no fusion at any point. 



In the fusions of the larvae so 

 far described, the archentera were 

 either independent or partially 

 fused. In the following examples 

 the two larvae are more completely 

 fused together, and there is but 

 one archenteron, the one belong- 

 ing to B having been either sup- 

 pressed or fused completely in 

 that of A. 

 Figure 12 is an example of a very complete fusion of two larvae with but 

 one archenteron. Detailed inspection shows that one larva is complete 

 and perfect, while the other has but a short blunt arm as the external 

 evidence of the second larva. Within this arm there is a normal and char- 

 acteristic ventral body rod with a number of hypertrophied processes, a true 

 dorso-ventral connective and an unusually thin ventral arm rod, and an 

 accessory dorsal arm rod. In this larva the right or the left half of the 

 larva has been differentiated, but less completely than the half larva of 

 figure II. 



Figure 13 resembles the fusion shown in figure 12 in several essential 

 points. Each fusion has but one archenteron, a dominant and perfect 



v.a.r. 



Fig. II. 



