The cephalochordate genera Branchiostoma and Asymmetron diverged during the Mesozoic Era.
In comparison to the purebreds, the hybrids were more refractory to fertilization, so enough sperm were added to each petri dish to turn the water distinctly cloudy (cephalochordate eggs are not susceptible to polyspermy).
In addition, cephalochordate larvae have several quantifiable phenotypic characters (like size of the preoral ciliated pit and angle of deflection of the larval gill slits and anus from the ventral midline).
The present discussion calls attention to the importance of settling the current disagreement about the course of evolution within the cephalochordates. The controversy could probably be resolved by adding nuclear genes to the phylogenetic analysis of the group and by sequencing the genome of a species of Asymmetron for comparison with that of Branchiostoma floridae (Putnam et al., 2008).
The production of concentric fertilization envelopes has not been reported for any other cephalochordate, or indeed for any kind of animal.
notochord is one example of an evolutionary novelty because it is made up of muscle cells that are innervated through contact with the ventral aspect of the nerve cord (Flood, 1975).
These data clearly demonstrate that Hb capable of reversible oxygen binding is expressed in two species of cephalochordates, thus expanding the known range of expression among chordates to include nonvertebrate chordates.
The existence of intracellular Hb dimers in cephalochordates is not unexpected because their nearest vertebrate relatives (the lamprey) also produce Hb dimers (12,13,14).
In future studies, we should use these probes to isolate endostyle-specific genes from cephalochordates, cyclostomes, and hemichordates.
In addition to zones 2, 4, and 6 that secrete mucus, zones 8 and 9 are of special interest because other research has indicated that the vertebrate thyroid gland may be homologous with the endostyle of tunicates (Dunn, 1974; Fujita and Sawano, 1979; Kobayashi et al., 1983), cephalochordates (Tsuneki et al., 1983; Fredriksson et al., 1985; Ericson et al., 1985), and larval lampreys (Egeberg, 1965; Fujita and Honma, 1969).