The minor coelomate phyla

Abstract

Three major protostome phyla, Annelida, Arthropoda, and Mollusca, have already been discussed. However not all the invertebrate adaptive lines resulting from the development of a coelom are represented by these groups. There are many minor phyla, which, whilst they may include a few extremely common species, are generally not abundant in terms of either species or individual number. The majority have neither economic importance nor ecological significance and the study of these animals has been largely neglected. However they do represent evolutionary divergences at the annelid-arthropod level of organization and as such are of interest to students of animal diversity. In ecological terms these minor phyla are not successful, and the animals tend to be found in the poorer and less accessible habitats. They may be conveniently considered under the following headings: the minor protostome coelomates, the lophophorate phyla, and the minor deuterostomes. The minor protostome coelomates include six phyla: Sipunculoidea, Echiuroidea, Priapuloidea, Tardigrada, Pentastomida, and Pogonophora. The lophophorates are a group of four phyla (Phoronida, Brachiopoda, Entoprocta, and Bryozoa) which all share the same type of food collecting organ, the lophophore. The minor deuterostomes will be considered later, after Echinodermata, the major phylum of invertebrate deuterostomes.