Introduction to the Deuterostomia Your mouth comes second. . .

The Deuterostomia comprise one of the major groups within the animal kingdom, and it is probably the most familiar to you, since you are a deuterostome! Most of the deuterostomes belong to one of two groups that include the majority of its members -- the echinoderms (the spiny skinned starfish, sea urchins, and their relatives) and the chordates (which include fish and other vertebrates). It may be surprising to some that chorates and echinoderms are related -- most people do not feel close kinship with sea urchins and sea cucumbers -- but both groups share a number of important morphological and developmental features.

In turn, the deuterostomes belong to a larger group within the Animalia called the Bilateria, because they are bilaterally symmetrical with a left and a right side to their bodies. Obviously this is not true for adult echinoderms like starfish, and you may wonder what radially symmetric critters are doing in this group. The answer lies in the development of the embryo. If you were to watch an embryonic starfish develop, you would see that it begins life bilaterally, but switches to radial symmetry as it matures.

The name deuterostome means "mouth second", and refers to one important developmental feature unique to this group. To understand this feature, a little animal embryology is necessary. In the earliest stages of embryo development, when there are only a few cells and the embryo resembles a tiny globe of cells, a small pucker develops on one side of the embryo. This grows into a pocket, and allows some cells to migrate inside to form an additional layer of cells within the outer layer. At this stage, the embryo is known as a gastrula. In the Protostomia, which is the other major group of the Bilateria, the mouth develops from the edge of this pocket, where the inner and outer layer of cells meet; the anal opening develops later. In the Deuterostomia, the reverse is true; the pocket edge develops into the anus, and the mouth is formed later. Hence the byline at the top of this page: "Your mouth comes second."

The phylogeny above is based on a combination of morphology and 18S RNA. It is not the final word on the relationships between these groups, and there are many competing hypotheses. For now, we prefer this grouping based on the available evidence, but as data continues to accumulate our picture of deuterostome relationships may change. The molecular phylogeny does agree in one respect with the morphological one in that there are close similarities between the larvae of hemichordates (acorn worms and pterobranchs) and some echinoderms. Hemichordates also share certain typical chordate features, such as pharyngeal slits, though they lack a true notochord and hollow nerve cord. A few paleontologists would place the Echinodermata closer to the Chordata or even within the Chordata, based on some unusual early echinoderms (the Homalozoa) that have been interpreted as having a tail and pharyngeal slits.

The cladogram above shows the major deuterostome groups. The groups Urochordata, Cephalochordata, and Vertebrata are all currently considered chordates, which has a page as well. Click on any box containing a picture to learn about that particular group.

Wada, H. and Satoh, N. 1994. Details of the evolutionary history from invertebrates to vertebrates, as deduced from the sequences of 18S rDNA. Proceedings of the National Academy of Sciences of the USA 91: 1801-1804.