Abstract:
The anatomy of Evechinus chloroticus (Valenciennes, 1846) is compared with that of Heliocidaris erythrogramma and with published descriptions of Echinus esculentus. The differences between the three species are slight, but uphold the view that Evechinus and Heliocidaris should be placed together and apart from Echinus and its allies. H. L. Clark (1912) first referred Evechinus to the Family Echinidae, and later (1925) referred both Heliocidaris and Evechinus to the Family Strongylocentrotidae. Heliocidaris, which has polyporous ambulacral plates and a circular ambitus, obviously conformed to Clark's Family Strongylocentrotidae. Evechinus, although oligoporous, was included because of the specialization shown by the larval form and the pedicellariae. Mortensen (1943) contends that both genera should be placed in the Family Echinometridae on account of the strongly developed single lateral tooth of the gemmiform pedicellariae, the paired nature of the poison glands, and the structure of the larval form.
Evechinus shares with Heliocidaris:
(1) The large collateral canal, which is conspicuous in both forms; it has, however, also been described from Echinus, although not from all members of the Echinidae (Bonnet, 1925).
(2) The gonads elongate from apex to lantern, although those of Evechinus are strongly coalesced while those of H. erythrogramma remain quite separate.
(3) The genital papillae, which are of very similar form in the two genera; the genital pores are also of the same order of size.
(4) A well defined ridge on the internal surface of the apical plates; in Evechinus the apical connection between the axial organ and the stone canal does not appear to be developed, although it has been described for Echinus (Chadwick 1900 et al.).
(5) The alimentary canal which is voluminous and greatly convoluted in both species, unlike that of Echinus; both forms bear well developed processes on the epiphyses of the lantern, which in those members of the Echinidae which possess them are only slightly developed (Mortensen, 1943); there are also small differences between Echinus and Evechinus in the structure of the pharynx, and in the histology of the alimentary canal.
Therefore it is considered that Evechinus and Heliocidaris are closely related genera, and although both Clark's and Mortensen's classifications have been admitted by their authors to be artificial, it is thought that Mortensen's criteria provide a more convenient basis for separation at the familial level than do those of H. L. Clark.
