A Generic Revision of the Stylasteridae (coelenterata: Hydrozoa). Part 3. Keys to the Genera

Dichotomous and tabular keys to the 23 genera, 2 subgenera, and 5 species groups of the Stylasteridae are presented. The keys stress easily recognized external skeletal characters, most of which can be seen using a dissecting microscope, such as: coordination of dactylopores and gastropores, presence or absence of gastroand dactylostyles, cyclosystem orientation, dactylopore spine shape, ampuUar position, gastropore tube shape, dactylopore tube length, and coenosteal texture. Each of these characters is briefly discussed in the context of its discriminating power and its phylogenetic value. Two new terms are introduced to characterize dactylopore tubes: axial and peripheral. Numbers of valid species for each genus and their geographic and bathymetric ranges are listed. Two hundred twenty-four stylasterid species are considered to be valid, including 24 exclusively fossil species. Stylasterids are cosmopolitan, but insular, in distribution, ranging from 0-2,789 m. This is the third of an originally conceived two part series, the first part being the description of the stylasterid genera (Cairns, 1983b), the second part a phylogenetic analysis of those genera (Cairns, 1984). However, faced with the need to identify a large collection of stylasterids following a 2-year hiatus from their study, I found it useful to construct and herein publish both dichotomous and tabular keys to the genera of the family. Such a tabular key could have been extracted from the coded data matrix published in part 2 (Cairns, 1984: 52-53, appendix 2); however, those data were not presented in a fashion that allowed easy interpretation or ready use as a key. The aim of these keys is to provide an efficient and accurate method to identify and compare stylasterid genera using the most easily recognized characters. The characters used are therefore not necessarily the most important phylogenetically (Cairns, 1984; 1987), but those that group or distinguish genera or groups of genera based on obvious, easily recognized characters. The keys are therefore artificial keys, but reflect in their organization many of the phylogenetic divisions suggested by Cairns (1984; 1987). The order of the genera in both keys is therefore roughly phylogenetic, in descending order of degree of derivation. DISCUSSION OF CHARACTERS USED IN THE TABULAR KEY The order of the characters used in the tabular key was determined both by their ability to discriminate taxa and their ease in recognition: the more conservative and easily recognized characters are listed from left to right. Characters 15 and 10 are all based on external skeletal characters; characters 6-8 require observations of a cross or longitudinal section of a branch; and character 9 is best viewed with the aid of scanning electron microscopy. Each character will be briefly discussed; illustrations and definitions of all structures can be found in Cairns (1983a; 1983b; 1986a; 1986b; Table 1). The increase in coordination of gastropores and dactylopores is considered to be a major evolutionary trend within the Stylasteridae (Cairns, 1987): a lack of coordination is considered to be the least derived state and the cyclosystem arrangement the most derived. The arrangement of dactylopores and gastropores