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CAMBRIAN PERIOD

542-488 million years ago

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tail

Urochordates were the first animals to develop a postanal tail which is considered one of the diagnostic features of chordates. Some of the hormones controlling the digestive tract in higher vertebrates are shared by urochordates.

In most invertebrates, the digestive system stretches the entire length of the body and ends at the tip of the abdomen. In chordates, there is a muscular tail which exists behind the anus through which the digestive tract does not pass. In the image of the lancelet above, the tail extends beyond the anus and digestive tract (the dark red tube marked by the line). It seems that the zinc finger transcription factor Manx was critical in the evolution of chordate tails, given that its mutation can cause tailless conditions in both urochordates and vertebrates (Satoh, 1995).

While the intestinal wall of urochordates is typically smooth, a few species possess grooves. Intestinal cells include absorptive cells, endocrine cells (whose secretions include gastrin, pancreatic polypeptide, and secretin), and mucus cells. There is very little muscle along the GI tract and most material is moved through ciliary action (Burighel, from Harrison, 1997, p. 256)