These, however, are apparently not expressed during head regeneration of Hydra . Several possible interpretations, emphasizing either evolutionary changes in gene usage or of the developmental program of Hydra itself, since it is a highly derived cnidarian, are possible. Similarities at the molecular and biochemical levels between hydroids extend beyond transcription factors and signalling. One class of these similarities involves cell-cell matrix interactions and the molecules that mediate them. In composition and behavior, cell-cell matrix interactions clearly display a degree of complexity that might not have been expected in these animals.
Complexity is also a feature of the neural system.
Hydra and other cnidarians also have a primitive immune system, with several constituent elements. In addition, he is examining the response to microbial infection and has preliminary evidence for IgG related genes and a rel-system, similar to that of insects and mammals. Part of the molecular complexity of cnidarians involves, of course, distinctive aspects of their biology. One of these is the production of a large number of signalling peptides. Over 550 peptides produced by Hydra have been identified, most of which are produced as distinct entities rather than as degradation products.