Note
As most of the current work on extracellular matrices is being carried out on vertebrate systems, it may be interesting to put this work in perspective with an examination of matrices in lower animals. Very little is known about the chemistry of matrices in invertebrates, but comparative fine structural studies permit some general statements regarding matrix topography. Matrix systems probably arose early in metazoan evolution, if, indeed, they did not make that evolution possible. Topographically, matrices may be divided into apical and basal types, each with distinct functions and different attractions for the evolutionary biologist.