Fig. 273.—Calyx of Actinocrinus, one of the Camerata, broken open to show structure. amb, Ambulacral groove enclosed in covering plates; B, basal; R1, R2, R3, the three radials of a column. (After Zittel.).
Fam. 5. Comatulidae.—Stem in the adult broken off, leaving only a stump, the centro-dorsal, covered with cirri. Six genera. Antedon (= Comatula) has already been described; many tropical species have numerous arms and often side-plates and covering plates. Actinometra is distinguished by its excentric mouth, and by the fact that the centro-dorsal is flat and has cirri only round its edges; Atelecrinus has an acorn-shaped centro-dorsal, and the basals are externally visible; Eudiocrinus differs from Antedon only in having five arms; Promachocrinus is a remarkable form, having ten radii (this is a unique feature in Crinoidea); finally, Thaumatocrinus has basals externally visible, large persistent orals and interradial plates, and in addition a short free appendage of several plates on the anal interradius. Antedon and Actinometra are almost world-wide. Six species of the first have been recorded from British waters, of which the commonest is Antedon rosacea; four others are distinguished by having longer cirri, and do not seem to be well defined; but A. eschrichtii, a northern form, is larger, and is distinguished by having long proximal pinnules. The other genera are rare, and occur in deep water.
When we turn to survey fossil Crinoidea, we are met with a bewildering variety of forms ranging from the Lower Cambrian to the present day. As already mentioned, there is no agreement amongst experts as to how they should be classified. Bather makes the fundamental cleavage depend on the possession of two whorls of plates in the base (Fig. 274), or of only one whorl. These two divisions he calls Dicyclica and Monocyclica respectively. He admits that in many forms allied to Dicyclica the infra-basals have disappeared; these he terms "pseudomonocyclic" forms, and believes that he is able to discriminate them from true Monocyclica.
Fig. 274.—Crotalocrinus pulcher. × 1. B, basal; Br, arm-fan of adhering branches; col, ossicle of stem; IB, infra-basal; R, radial. (After Zittel.)
The present author is utterly unable to believe that the Crinoidea diverged into two groups on what is a trifling point of meristic variation comparable to the varying number of rows of plates in the interradial areas of the older Echinoidea; and he is equally sceptical as to the validity of Jaekel's division of the group into Cladocrinoidea and Pentacrinoidea, leading to the view that organs like pinnules represent totally different structures in different groups. Wachsmuth and Springer adopt as bases of classification the extent to which the arms and their branches are incorporated in the disc, and they recognise three main divisions: Inadunata, in which the arms are completely free from the calyx; Articulata, in which the arms are partly incorporated but the tegmen remains flexible; and finally Camerata, in which the arms and their first branches are largely incorporated in the cup; the tegmen is converted into a rigid dome and the ambulacral grooves on it become closed, as does the mouth, by the meeting of overarching folds; the grooves remaining, of course, open in the distal portions of the arms (Fig. 273). This classification, founded as it is on physiological factors, seems to the present author more satisfactory. Speaking generally, the points in which fossil Crinoids may differ from living genera are: (1) the total absence or irregular nature of the branching in the arms, so that pinnules may be said to be absent; (2) the closure of the ambulacral grooves and mouth already alluded to, and (3) the adhesion of the arms in the same ray to produce net-like structures (Crotalocrinus, Fig. 274), or a fan-shaped structure (Petalocrinus); (4) the frequent presence of two rows of brachials in one arm (biserial structure); (5) the development of an enormous anal tube, so large that in extreme cases (Eucalyptocrinus) the arms may be lodged in grooves of it.
CLASS II. THECOIDEA (EDRIOASTEROIDEA, Bather)
These remarkable Pelmatozoa are the most primitive known. They have sac-like or sometimes cushion-shaped or even disc-shaped bodies, covered with numerous irregular plates without any symmetry in their arrangement. There is no stem, but when they are fixed this is effected by an adhesion of the aboral pole. There are no arms, but on the upper surface is to be seen the impression of five ambulacral grooves radiating from a central mouth. These grooves are bordered by covering plates, which in the earliest form (Stromacystis) are seen to be slight modifications of the plates covering the upper surface of the body, but in the later genera (Fig. 275, Thecocystis) become specialised. The anus is situated on the side, as is also the madreporite. It has been suggested that Eleutherozoa were derived from this group; that individuals were occasionally overturned by the waves or currents, and in this way compelled to use their podia for locomotion. When Eleutherozoa, however, have a fixed stage in their development, they are fixed by the oral, not the aboral, surface, and hence can have no close affinity to Thecoidea. Thecoidea begin in the Middle Cambrian, but according to Jaekel impressions in the Lower Cambrian, referred to Medusae, may be casts of this group.
