Taxonomy and distribution of Australian cephalochordates
(amphioxus) embryos: procurement, culture, and basic methods.
At present, this fundamental disagreement impedes speculation about cephalochordate
(and even chordate) evolution--even if one ignores the added difficulty that a representative of an anciently diverged lineage may have lost some original characters and evolved some highly derived ones.
Our data also provides information on the relative importance of these three groups in comparison with other main macro-invertebrates from the Punta Morales flat collected during the 1984-1987 survey, like cephalochordates
(Vargas & Dean, 2010), mollusks (Vargas-Zamora & Sibaja-Cordero, 2011), echinoderms (Vargas & Solano, 2011), crustaceans (Vargas-Zamora, Sibaja-Cordero, & Vargas-Castillo, 2012) and polychaetes (Vargas-Zamora, Sibaja-Cordero, Dean, & Solano-Ulate, 2015).
Although several hypotheses were postulated to explain the origin of protamines in vertebrates (see the following), there were no studies at that time on the SNBPs of other zoological groups with close affinity to the vertebrates, such as chordates or cephalochordates
, to help understand the problem.
Ascidians are members of the subphylum urochordates, which is a group of the phylum chordates along with cephalochordates
and vertebrates (Satoh, 2003).
Recent sequence-based phylogenies place cephalochordates
(amphioxus) as the sister group of all other chordates and Xenoturbella as a new phylum related to hemichordates and echinoderms (Bourlat et al.
In contrast to urochordates, cephalochordates
have retained many features of the basal chordate body plan and are likely to approximate the ancestor of both urochordates and vertebrates.
It is present in urochordates, cephalochordates
, and larval lampreys, and its function is secretion of a mucous net to retain particles during feeding (Olsson, 1965; Deibel and Powell, 1987; Hiruta et al.
, urochordates, hemichordates, and echinoderms); but complete genome sequences have not yet been obtained for species representative of these phyla.
The myotome tissue of cephalochordates
consists of chevron-shaped bundles of cells, and the notochord tissue consists of a lamellar structure of stacked cells surrounded by a fibrous sheath.
The transmission electron micrographs suggest that the contractile vessel-like structure in the ventral midline of larval cephalochordates
is not the sub-intestinal blood vessel, but rather the ventral coelom.