. Carnegie Institution of Washington publication. 114 Papers from the Marine Biological Laboratory at Tortugas. 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 severa
. Carnegie Institution of Washington publication. 114 Papers from the Marine Biological Laboratory at Tortugas. 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 point
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