Below the twelve models in Case 36 may be seen a representation of three stages in the history of the Feather-star (Antedon bifida). The larvæ of this Echinoderm are not free, but are attached by a stalk (Fig. 6); in the common Feather-star and other Comatulidæ the stalk is found during larval stages only; in others, such as Pentacrinus, it persists throughout life.
The presence or absence of this stalk has been taken as the first character of importance in the classification of Echinoderma which may be divided into two groups:—
A. Pelmatozoa,[[24]] or Echinoderms provided with a stalk throughout life or in the larval stages only. To this group belong the Crinoidea, and the extinct Blastoidea, and Cystidea.
Fig. 6.
Pentacrinoid stage of Antedon rosacea.
a, arms; b, basals; r, radials; s, stalk.
B. Echinozoa, or Echinoderms without stalks at any time of their existence. To this group belong the Asteroidea, Ophiuroidea, Echinoidea, and Holothurioidea.
Crinoidea.—This Order may be described as stalked, globular, or cup-shaped Echinoderms, in which the oral surface of the calyx or disk looks upwards, and in which five jointed and generally branched rays arise from the central disk. Their joints have jointed pinnules at their sides, and the sucking-feet have the form of tentacles.
The stalked representatives of this Order are placed on tables and brackets near the south door, and are worthy of being particularly noticed for their fine preservation, size, and beauty. The largest specimen of Pentacrinus decorus was taken on a telegraph-wire, to the covering of which the stalk of the Crinoid is still attached. Metacrinus is a more lately discovered genus, which appears to be confined to the eastern seas.
A few dried unstalked Crinoids are shown in Table-case 1; these show the leading modifications of structure in the two great genera Antedon and Actinometra.
