Cephalochordata (Lancelets or Amphioxi)

Cephalochordates are swift, fishlike chordates that burrow in sand and filter suspended food particles in the seawater (Ruppert, Fox, and Barnes 2004a). Known as lancelets or amphioxi due to their elongated body with pointy ends, they exhibit several repeated traits along the anteroposterior axis, such as paired coelomic sacs, muscle bundles, gill slits, and gonads (Beklemishev 1969d).

Directly visible through the transparent adult body are the V-shaped muscle bundles arranged in a segmental manner along the anteroposterior axis and the repeated series of gonads located more ventrally (Bertrand and Escriva 2011). Curiously the left and right somites are out of register to each other (Ruppert, Fox, and Barnes 2004a). Underneath lies a pharynx with a series of gill slits separated by cartilaginous bars (Bertrand and Escriva 2011). Associated with the gills there are the paired nephridia also organized in a segmental manner (Goodrich 1902; Holland 2017).

There are no visible segmental traits in the nervous system except for serially repeated dorsal serotonergic neurons (Yasui et al. 1998; Candiani et al. 2001; Wicht and Lacalli 2005) which follow the out-of-register arrangement of the musculature (Wicht and Lacalli 2005). In addition, there are inner cholinergic and GABAergic/glycinergic neurons that are segmentally arranged in the hindbrain (Candiani et al. 2012).

The first segmental structures in the amphioxus embryo are mesodermal constrictions that mark the somite boundaries (Conklin 1932). These evaginations progressively pinch off from anterior to posterior forming 8-12 pairs of somites (Conklin 1932; Holland 2015), while subsequent somites arise instead from the tail bud (Holland 2015). The somites are further regionalized into three populations under distinct regulatory landscapes (Aldea et al. 2019), revealing a hidden complexity of molecular interactions and developmental processes that act in concert to generate a uniform series of repeated structures.

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