The cords give off branches in each joint, one pair to the thin aboral integument, two other pairs to the lateral and oral surfaces. Bipolar cells have, in some instances, been seen in the course of these branches. Their ultimate twigs appear to be connected with the muscles (brachial) through stellate cells and through other similar cells with the integument and with the tactile papillae furnished with sensory hairs on the tentacles. A set of branches or a plexus at the side of the ambulacral grooves in the disc and arms forms a parambulacral system with a longitudinal connecting nerve in Actinometra nigra. And in Antedon Eschrichti there is a circumoral labial plexus.

1 In consequence of this fact the visceral mass of the disc is easily detached, Hyponeme Sarsii (Loven), a supposed Cystoidean from Torres Straits, is the visceral mass of an Antedon common at Cape York.

The blood-vascular system consists of a circumoral ring with an ambulacral vessel underlying the nerve; of a dense circumoesophageal labial plexus connected to the oral ring and sending out branches to the genital plexus as well as intervisceral vessels; of a plexiform organ lying interradially in the disc anteriorly to the mouth and connected to the labial plexus, genital and intervisceral vessels, and extending into the 'chambered organ' of the calyx. This organ lies between or below (Comatulidae) the basals, and within the central portion of the aboral nervous system. It consists of five peripheral and radial chambers and a central plexus or I-2 vessels. Five vessels pass, one from each chamber, to the corresponding cirrus and are continued down the stem dilating in each nodal joint and sending off a branch to each cirrus. The plexus supplies the remaining whorls of centro-dorsal cirri in Comatulidae, and is continued down the stem in stalked forms. It has recently been shown that two vessels separated by a septum lie in the centre of the axial nerves of the arms.

The water-vascular system consists of a circumoral ring and radial vessels, all of which are beneath the corresponding blood-vessels. Calcareous subambulacral plates sometimes occur in the connective tissue under the radial water-vascular vessels. In some Palaeocrinoidea they were regularly arranged in two series. Ciliated branched water-tubes depend from the ring and origins of the radial vessels and open into the coelome. Water-pores, or short tubular canals with a median ciliated dilatation, open into the coelome from the exterior. Water-tubes and pores, taken together, correspond to the madreporic system of other Echi-noderms. In a young larval Antedon one tube and one pore are found - the latter in an oral plate - in the interradius to the left of the anus, the one which contains the madreporite in Asteroidea. In an older larva there is a tube and pore in each of the five interradii, a condition persistent in Rhizocrinus lofotensis. The water-tubes are numerous in other adult Crinoids, e. g. thirty in each interradius in Antedon rosacea. The water-pores are few and pierce the orals in Hyocrinus. In Antedon rosacea there are 1500 on the disc. They are relatively few in the anal interradius and in Actinometra are found on the arms and pinnules.

Here they open into the genital canal or coelomic space surrounding the genital rhachis. When the disc is covered with plates the pores may be scattered singly or grouped up to the number of twenty in a plate. The radial water-vascular vessels give off alternately to the right and left, in groups of three each, delicate tubular branches, respiratory in function, which form the tentacles homologous with tube-feet. The vessel is slightly dilated on the side opposite to the origins of the tentacles.

The genital gland appears to consist of a central portion, not always traceable, within the labial blood-vascular plexus from which a branch extends along the arms and pinnules, surrounded by a plexus of bloodvessels, and contained in the genital canal a special offset of the coelome. These structures lie between the subtentacular and coeliac canals. The gland is essentially a tube lined by an epithelium, from which are derived spermatozoa and ova, according to sex. The greater portion is not fertile, and constitutes the rhachis. Fertile portions occur rarely in the disc and arms. In Holopus alone they are always in the arms; otherwise they are confined to the pinnules, where they form round or elongated masses. The duct in the female is a wide canal, in the male one or two fine canals to each mass. The mode in which the ducts are formed is unknown.

Sacculi or aggregations of colourless vesicles of unknown function are found in variable numbers at the side of the radial water-vascular vessels, and in Antedon in the walls of the digestive tube. They are absent in Actinometra, where, however, isolated vesicles may occur in the perisome. Two colouring matters giving special absorption bands are found in Crinoids, Pentacrinin from Holopus, Pentacrinus and Metacrinus, and Antedonin from three species of Actinometra. The yellow or rose-red pigment in Antedon gives no bands. The arms of Comatula and some other Crinoidea may become locally deformed by the attacks of the Myzostomidae, a parasitic order of Chaetopoda; see p. 609. Extinct species seem to have been similarly attacked (von Graff).

Holopus and Hyocrinus are abyssal forms (1200-2500 fathoms); an Antedon has been dredged at 2900 fathoms, but the majority of Crinoidea occur in shallower waters. They are generally met with in masses, as is the case with Crinoids in the Silurian, Carboniferous, and Oolitic rocks. .Antedon is a cosmopolitan genus; Holopus is confined to the Caribbaean sea. Stalked Crinoids are found between the parallels of 68 N. and 46 S. latitude; Comatididae between 81 N. and 52 S. The Holopodidae originate in the Upper Silurian, Pentacrinus in the Trias, Antedon in the lower Oolite. Many fossil Comatididae are large in size.

The gastrula mouth in Antedon closes early, and is not terminal and posterior, as in other Echinodermata, but behind the larval third ring of cilia, on a flat ventral surface (cf. p. 548, ante). The water-vascular vesicle is formed after the left and right coelomic vesicles, which become placed terminally, the left orally, the right aborally at the base of the stem into which it extends. The water-vascular vesicle grows round the oral end of the archenteron, and subsequently divides the left peritoneal vesicle into an anterior and posterior portion. The former forms the oral vestibule, enclosed within the oral plates. The mouth is formed in its floor, and the ambulacral grooves grow out from it radially. Hence their epithelium is hypoblastic in origin. The posterior section sends out the subtentacular canals, as the right vesicle does the coeliac. The water-vascular ring developes ten pairs of tentacles within the vestibule. Five pairs are simple and short, and are interradial like the oral plates. The other five are radial, and consist each of three tentacles, the central one of which is carried outwards in the growth of the arm.

The five orals are at first large, and one is pierced by the primary water-pore; they are eventually resorbed as is the anal interradial plate, which at first touches a basal, and is afterwards raised on to the disc.

The Crinoidea are divisible into

1. Palaeocrinoidea (= Tesselata): calyx not invariably pentamerous; ategmen calycis generally present; anal interradius large, with an anal interradial touching a basal. Palaeozoic.

2. Neocrinoidea (= Articulata + Holopus 4- Marsupites): calyx pentamerous; interradials rarely intervene between the radials; orocentral always absent; orals, when present, surround the mouth. Mesozoic and recent. Comatulidae and other living Crinoidea.

General anatomy, P. H. Carpenter, Challenger Reports, xi. 1884. New sp. of Metacrinus, Id. Tr. L. S. (2), ii. (14), 1885. Variations in structure of arms, Id. ibid. ii. (17), 1886.

Fossil forms. See in addition to Zittel's work, cited p. 549, ante, de Loriol, 'Palaeontologie Francaise,' Paris, x., pt. 1, 1882-84; Wachsmuth and Springer, 1Revision of Palaeocrinoidea) Proc. Acad. Nat. Sc. Philadelphia, 1879, 1881, 1885; cf. P. H. Carpenter, supra, and in A. N. H. (5), xvii. 1886, p. 276.

Deformities in fossil Crinoids due to Myzostomidae, von Graff, Palaeonto-graphica, xxxi.

Antiambulacral nervous system, Milnes Marshall, Q. J. M. xxiv. 1884.

Process resembling copulation in Comatula, Jickeli, Z. A. vii. 1884 (A. N. H.

(5), xiv-).