The higher multicellular animal world can be divided into two major groups by their embryological development. The majority are protostomes in which the blastopore in the original ball of cells, the blastula, becomes the mouth of the developing organism. The Arthropods, Annelids and Molluscs are all Protostomes plus all the pseudocoelomate phyla. The rest are Deuterostomes, animals in which the original blastopore closes and another pore opens to become the mouth. We are deuterostomes as are all the members of our phylum, the Chordata. Starfish, sea urchins and sea cucumbers, of the phylum Echinodermata, are also deuterostomes. In between is a small group of phyla known as the lophophorate coelomates which show characteristics of both protostomes and deuterostomes and may be descendants of the ancestors of the original deuterostomes.
The Phoronids are lophophorate coelomates, along with the Ectoprocts and the Brachiopods. All three have a true body cavity or coelom and they also have a crown of ciliated tentacles that is called a lophophore. Each phylum has a distinct life style and body plan but the shared lophophore shows that they are related to one another. Phoronid mouths develop from the blastopore, which relates them to the protostomes but in other characteristics they are obviously closely linked to the deuterostomes. Both spiral and radial cleavage is seen in the phoronids and cleavage is indeterminate, which is a deuterostome characteristic. In protostomes, the coelom develops from mesoderm and is termed a schizocoel, while in deuterostomes, the coelom is an enterocoel deriving from gut cells. In the phoronids and other lophophorates both schizocoelous and enterocoelous events are seen during the formation of the coelom.
As Meglitsch points out (p.672) “…the preponderance of evidence points to the lophophorates as deuterostomes and other than the mouth formation of phoronids the protostome-like traits are either tenuous or subject to argument. Certainly if one looks for relatives, it is far easier to see them as ancestral to other deuterostomes than as being derived from some known stock of protostomes… If they are indeed the most primitive deuterostomes, it is to these rather inconspicuous and more or less neglected animals that we must turn to understand the origin of one of the main branches of the animal kingdom…” and indeed our own branch.
The phoronids are basically marine, tube-dwelling worms. They have a long trunk with a bulbous back end and at the head end, the ring of tentacles with a central mouth. The gut is U-shaped and the anus also is at the head end near the mouth. They live in tubes in shallow bottom sediments and use their tentacles to filter feed. There are only ten described species but they can be quite numerous in the right habitat. Their common name, horseshoe worms, comes from the horseshoe-shaped lophophore. They secrete the tubes that they live in, gluing together the sediments to make them. Often many worms, each in its own tube, are found in one area. Phoronids can also secrete acids that dissolve rocks so they can make their tubes in rock if necessary.
Most species are hermaphroditic although a couple have separate sexes. Fertilisation is internal and the eggs are brooded. The larvae may be brooded or released into the water. After a short free-living stage, the larvae sink to the bottom and construct the tubes in which they will spend the rest of their lives.
References: Meglitsch, P. 1972. Invertebrate Zoology. Oxford University Press.