Chordates & Craniates
Fishes are vertebrates and belong to the subphylum Craniata of the phylum Chordata. This phylum also includes two other subphyla, the Urochordata and the Cephalochordats, which have been treated in the Guide to Invertebrate Animals. Chordates are eucoelomate animals, that is their main body cavity, containing most of the large organ systems, is a true coelom. In this respect they resemble a number of invertebrate groups, such as the annelids and echinoderms. Again, in all chordates there is a characteristic deuterostomous pattern of embryological development in which the anus forms at the site of the blastopore. This is also a feature of the echinoderms, hemichordates and Chaetognatha. It contrasts quite sharply with the proterostomous development (where the blastopore becomes the mouth) of most invertebrates, such as annelids, arthropods and molluscs. Thus in their anatomy and embryology the chordates are linked in various ways with the lower phyla.
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