54. Regarding the two main branches of the animal tree, what is the difference between the Arthropod line the the Vertebrate line?
In the formation of the embryo, which starts off as a hollow ball of cells, the ectoderm begins to invaginate inward to form a gut. In the Arthropod line (on the left) the first opening becomes the mouth (protosome line), in the Vertebrate line (on the right) the first formed opening becomes the anus and the second opening becomes the mouth (deuterostome line).