555-545 million years ago


Hemichordates share a number of additional features with vertebrates: striated muscle, a possible homolog of the thyroid gland, podocytes in the urianry system, white blood cells, and the first complement cascade which vertebrates would elaborate in their immune responses.

Most hemichordate muscle is smooth but some striated muscle also exists (Benito, form Harrison 1997, p. 44). The glandular region of the ventral pharynx seems to correspond to the endostyle of lancelets which, in turn, is considered to be the homolog of the vertebrate thyroid gland (Benito, form Harrison 1997, p. 72.).

The urinary system of hemichordates is similar to that of vertebrates in a number of characteristics. In hemichordates, blood arrives under pressure (from heart contractions and contraction of the lining of the dorsal blood vessel) to thin walled sinuses. Here, podocyte cells send their pedicel processes to form slit membranes which filter the blood (similar to the situation in higher vertebrates). The filtrate, or primary urine, may be processed through the reabsorbtion of ions and other materials by podocytes and blood cells before becoming the secondary urine which is excreted from the body (Benito, form Harrison 1997, p. 64). Echinoderms and urochordate pterobranchs also possess podocytes (Stach, 2000).

In hemichordates, blood flows through spaces in connective tissue in which amoeba-like blood cells travel. The amoebocyte blood cells typically possess kidney-shaped nuclei. (Benito, form Harrison 1997, p. 61).

Although primitive invertebrates possess the domains used by the complement system, a complement system did not evolve until the deuterostomes. Primitive deuterostomes evolved the components of the complement cascade and the genome duplications early in the history of the vertebrates allowed the integration of several related pathways (Fujita, 2004). The complement pathway can also be activated by the serum mannose binding lectin (MBL, a C-type lectin) to the carbohydrates of microbes. The serine protease MASP (MBL-associated serine protease) activates complement C3 which can either result in opsonization or the assembly of the complement factors which put a hole in the microbial membrane (Vasta, 1999).
Sponge molecules possess the SCR/CCP domains which are found in complement proteins (Zarkadis, 2001). In primitive deuterostomes, it is not obvious that the lectin and alternative pathways have different functions (Zarkadis, 2001). Elements of the lectin pathway, MBL and MASP, are known since protochordates. Elements of the alternative pathway, C3 and factor B, are known in echinoderms and factor D in bony fish. (Zarkadis, 2001; Lundqvist, 1999;Pearce, 2001). The sea urchin protein SpBf is a complement protein which possesses SCR domains, a von Willebrand factor domain, and a serine protease domain. Sea urchins possess complement C3 proteins which seem to function in opsonization and whose levels increase in response to infection. Sea urchin proteins possess a thioester region, which in vertebrate complement proteins C3, C4, and C5, is the region which are exposed and bind target molecules in complement activation. Some insect proteins have molecules similar to complements with thioester regions (Smith, 2002).