THE DIGESTIVE SYSTEM
Although the early vertebrates did modify their digestive
systems, they would not be dominant members of marine food chains until
the evolution of jaws. They were preyed upon by large invertebrate predators
such as cephalopods and sea scorpions.
In the early vertebrates (and perhaps the primitive craniates as well),
the coelom formed from mesoderm after it separated from other tissues
during development rather than form from segmented pouches from the gut
as in echinoderms and lancelets) (Romer, p. 315). Primitive vertebrates
evolved oral glands, some of which secreted mucus (Romer, p. 329). Other
changes include the reduction in the number of pharyngeal pouches (Weichert,
1970, p.214), the secretion of amylase from the pancreas (Stevens, p.160),
and the function of pancreatic lipase as the most important enzyme in
the digestion of lipids (Stevens, p.168).