[289] It is figured in my South D. Foss. pl. xvi. fig. 6.
The Marsupite is an exceedingly interesting type, in a zoological point of view, since it forms a link that unites the Crinoidea with the Comatulæ, or feather-stars, which we shall presently notice. Its general form and habits are sufficiently indicated by the numerous specimens that have been collected in the Chalk of the south-east of England.
The body of the animal was inclosed in a crustaceous case formed of large plates, articulated to each other by suture; the mouth or oral aperture was situated in the centre of the plated integumental cover of the receptacle. The organs of locomotion and prehension consisted of five arms or brachial appendages, formed of ossicula as in the crinoidea, and the whole was invested with soft tissue or integuments. When floating in the water, the creature could spread out its tentacula, and form a net to capture its prey, and by closing them, secure it, and convey it to its mouth. The figure, [Lign. 96], is restored from specimens which separately exhibit the parts here represented in connexion.
Fossil Asteriadæ.
FOSSIL ASTERIADÆ.
The radiated animals, popularly called Star-fishes, from their stellular forms, are so abundant on our coasts, that the common five-rayed species must be familiar to all my readers who indulge in rambles on the sea-shore, and will serve as an illustration of the general appearance and structure of the beings whose petrified remains now claim our attention. This species belongs to the division of Asteriadæ, in which the rays are elongated, and far exceed in length the diameter of the disk; in another group (Goniastea, or Cushion-star), the body is angular, and the lobes or rays are short, and not longer than the diameter; while in a third subdivision (Comatula and Ophiura), the arms are distinct from the body, and articulated, elongated, and ramified, as in the Crinoidea.
The external surface of the common Star-fish is soft, and attached to a tough coriaceous integument, investing a skeleton composed of an infinite number of calcareous ossicula, arranged in regular series along the margins of the rays. Each ray has a longitudinal furrow, perforated at the sides by alternating rows of pores, through which tubular tentacula are protruded. The mouth is situated in the centre of the under surface. Now if we imagine a Star-fish placed with its mouth upwards, and the five rays fringed with long articulated tentacula, as in the Comatula, and fixed by the centre of its dorsal surface upon a jointed stem, we shall have the essential characters of a crinoidean; and the animals of one recent tribe of Asteriadæ are actually in this condition in the earlier stage of their existence: these are the Comatulæ, or Feather-stars, in some of which (the Euryale), the arms are as numerously subdivided as in the Pentacrinites.[290]
[290] The reader interested in this subject should peruse the charming volume on British Star-fishes and other Echinoderms, by Professor Edward Forbes. 1 vol. 8vo. John Van Voorst. 1841.