There is no doubt that it is by the above arrangement that the living Crinoids obtain their food, and the mechanism seems to have been essentially the same in many extinct species. In the Paleozoic Crinoids, however, there seems to have been a modification of this arrangement. In these forms, as in Actinocrinus (fig. 75), the arms have much the structure of those of the recent Crinoids, and are deeply grooved on their ventral surfaces. The ventral surface of the calyx, however, exhibits no central aperture, but only a proboscidiform tube, which arises from one of the inter-radial spaces (i.e., one of the intervals between two of the arms). This tube is often of great length, and a good deal of controversy has taken place as to its nature. Without entering into the conflicting views upon this subject, it may be stated that the preponderance of authority is in favour of the view that this "proboscis" is an anal tube, having the vent at

its extremity. Good observers, however, regard it as discharging the functions of both the mouth and the anus. Be this as it may, the grooves on the ventral surfaces of the arms are certainly not continued over the ventral surface of the calyx, but, on the contrary, stop short at the bases of the arms. Their further course was long a mystery; but it is now known that they are continued below the ventral surface of the calyx as a series of covered passages or tunnels, the external apertures of which are placed at the points where the arms spring from the disc (see figs. 74-76). These covered channels are either simply roofed over by the calcareous integument of the calyx, or are rarely excavated in its walls; and they converge to a central point in the middle of the ventral surface of the disc. Here, it is believed, is placed the mouth, concealed by the calcareous plates of the perisome. From the known function of the brachial grooves in the living Crinoids, this view would

seem to be the most probable one; but, as before remarked, good authorities regard the excentric "proboscis" as the mouth.

In another group of Crinoids, represented by the living Feather-stars (Comatula) and the extinct Saccosoma (fig. 77),

Fig. 77.-Saccosoma pectinata, a free Crinoid. Jurassic.

the animal is only stalked when young, and in its adult condition leads a free life. The young form in the members of this group is supported upon a jointed calcareous column, by which it is fixed to some foreign object; and at this stage it in no respect differs from the ordinary stalked Crinoids.

At a

66

certain period of its existence, however, the calyx drops off its column, and becomes a locomotive animal. It now has a near resemblance to one of the Brittle-stars (Ophiuroidea); but is distinguished, not only by its developmental history, but by the possession of lateral pinnæ to the arms, and in having the reproductive organs situated external to the body proper. In the Feather-stars (Comatula or Antedon) the dorsal surface of the disc, at the point where the column was originally inserted, carries a series of jointed filaments or cirri," by which the animal can moor itself to any foreign object. These may be regarded as homologous with the "side-arms" of the column of certain Crinoids. When the animal is thus temporarily moored by its dorsal cirri, it is placed in the ordinary position held by the Crinoids-namely, with the mouth and ventral surface of the disc looking upwards. When creeping about, on the other hand, by means of the long and flexible arms, the animal occupies the position held by the Star-fishes and Ophiuroids-namely, with the mouth and ventral surface of the disc directed downwards, or towards the ground.

Having now given a general account of the structure of the Crinoids, it remains to consider some of their parts in greater detail. In the first place, as regards the "column," we find that the stem of attachment is composed of a great number of separate plates or "articulations" placed one above the other, and so jointed to one another that whilst the amount of movement between any two pieces must be very limited, the entire column acquires more or less flexibility. The column is perforated by a round or five-sided canal which pierces every joint, and runs along the entire length of the stem. In the Paleozoic Crinoids, with few exceptions, the column was round; but in Platycrinus it is oval or elliptical (fig. 73). In the genera Pentacrinus (fig. 78) and Extracrinus the column is pentagonal in outline; but much less markedly so in the former than in the latter genus. The joints articulate with one another by surfaces or facets which are differently marked in different cases. In the Paleozoic forms, as in Platycrinus (fig. 73), the articulating facets are marked by more or less numerous striæ which radiate from near the centre of the joint. In most of the Mesozoic genera, on the other hand, as in Pentacrinus (fig. 78), the articulating facets are united by crenated ridges arranged in a pentapetalous figure. In many cases, as in Extracrinus and Pentacrinus, the column is furnished with more or less numerous "auxiliary" arms, or “sidearms," the function of which is not altogether clear.

The dorsal surface of the cup or "calyx" is composed of a number of calcareous plates accurately fitted together. The number and arrangement of these vary much in different

Fig. 78.-Pentacrinus fasciculosus, showing the form of the column, and one of the facets of a joint. The central figure shows the arms, and the summit of the column with side-arms.

genera, and it will be sufficient to indicate here their general disposition. Reposing directly upon the summit of the column

is a series of plates which are termed "basal" from their position, and which constitute the "pelvis" of Miller. The "basals" may be five, four, or sometimes three in number, and they form the lowest portion of the cup. In some cases

the "basals" are succeeded by a second row or cycle of plates, which should properly be regarded, with Professor Beyrich, as a second series of basals, but which are sometimes regarded as something special, and are termed the "parabasals" or "sub-radials."

The basals (fig. 79, b) are succeeded by a

Fig. 79.-Diagram to show the structure of the calyx in the fossil Crinoids; b Basals; Radials; i Inter-radials; a Anal plates. Calyx of Forbesiocrinus. (After Pictet.)

series of two or three rows of plates, which are directly superimposed upon one another, and which form the foundations of the arms (r, r). These are termed the "radials" (the " COStæ" of Miller), and are termed "primary," "secondary," and "tertiary," according to their distance from the basals. The last radial plates, or those furthest from the column (sometimes called the "axillary radials"), give origin to the arms. The radial plates are arranged in a series of vertical columns, which extend from the summit of the basals to the bases of the arms. Between the different columns of radial plates, however, are intercalated certain other smaller plates, which