On Traces of Ancestral Relations in the Structure of the Asteroidea

Abstract

[Plate XV.] The Echinodermata constitute a sub-kingdom of animals in which the structure of the various divisions presents a number of highly remarkable points of affinity and difference. Indeed, so strikingly is this the case, that when typical or representative specimens are subjected for the first time to superficial examination by a person unacquainted with their internal anatomy, the principal groups would most probably be looked upon by him as perhaps the most diverse of creatures—popularly speaking—that it could be possible to link together. Five Classes of living Echinoderms are included in this category:— 1. The Crinoidea, which are familiarly known as ‘Sea-Lilies.’ 2. The Ophiuroidea or ‘Brittle-Stars.’ 3. The Asteroidea or ‘Starfishes.’ 4. The Echinoidea, or ‘Sea-Urchins.’ 5. The Holothuroidea or ‘Sea-Cucumbers.’ The present communication will deal with certain homologies that exist in the structure of the Asteroidea and Ophiuroidea, upon which deductions of considerable importance have been based. It will be desirable, however, to explain before proceeding further that although the word ‘homologies’ is employed in this instance as synonymous with ‘traces of descent,’ it is not the purpose of this paper to trench upon the subject of the phylogenetic development of the Echinodermata as a whole, but simply to confine the remarks to several points of evidence that indicate ancestral relations between the two groups in question. Although we shall not directly press into our service palaeontological evidence, I make bold to believe that the subject will not be without interest to Geologists; —first, from ...