Fig. 365.—Medusa-headed Pentacrinoid.
a. Crown and part of stem; b. Upper surface of body, the arms broken away, showing the food grooves passing to the central mouth.—(Warne.)
Just one other special feature should be noticed: radial canals pass along under the ambulacra, and join a ring-canal around the mouth, well supplied by nerve cells.
Fig. 366.
1. Transverse section of a branch of Myriapore; 2, and the others Section of the stem of Virgularia mirabilis; 3, Spiculum from the outer surface of Sea-pen; 4, Spicula from Isis hippuris; 5, from Gorgonia elongata; 6, from Alcyonium; 7, and from Gorgonia umbraculum; 8, Calcareous remains of a Crinoid.
Crinoids (stone-lilies), on the other hand, are formed of a series of flat rings, pierced through by a narrow canal. The ossicles, as they are termed, are joined by ligaments passing through their solid substance and endowed with muscular power; the central part serves for the passage of blood-vessels, and is surrounded by a sheath of nervous tissue that controls the movements of the stem, the latter being encrusted by a number of fine rootlets. The stems possess a limited power of bending. In the words of Professor Agassiz, “The stem itself passes slowly from a rigid vertical attitude to a curved or even a drooping position; the cirri move more rapidly than the arms, and the animal uses them as hooks to catch hold of objects, and on account of their sharp extremities they are well adapted to retain their hold of prey.” The rosy-feather star-fish is often found clinging to a tube of the Sabella worm; the food of crinoids consists of foraminifera, diatoms, and the larvæ of crustaceans. There are so many curious features in connection with the Echinodermata that my readers may with advantage consult “The Challenger Reports” and Warne’s “Natural History” on other points of interest.
Holothuroidea (sea-cucumbers) are elongated slug-like creatures, the skin being in structure similar to that of the slug, with a comparatively small amount of calcareous matter. Usually this occurs in small spicules, which assume very definite shapes, as the anchors of Synapta ([Plate IV]., No. 87, and in [Fig. 355]). There are also rings of calcareous plates around the gullet, five of which have the same relation to the radial water-vessels as the auricles round the jaws of a sea-urchin, and which likewise serve for the attachment of muscles. These plates are seen in [Plate VIII]., Nos. 171 and 172, as they appear coloured by selenite films under polarised light. Around the mouth in Cucumaria is a fringe of branched tentacles connected with the water-vascular ring; these appear to be used as a net to intercept floating organisms.
Correlated with the star-fishes is a small family based on the character of their pincer-like organs, called pedicellariæ, on the surface of the test (shown in [Plate IV]., Nos. 93 and 94, magnified × 25). Movable spines cover the surface of these echinoderms, varying in size from minute bristle-like structures to long rods. The pedicellariæ are, it is believed, derived from the smaller spines, and two of them are united at the base by muscles, slightly curved, and made to approach each other at their extremities. There is a gradual modification of this type through the whole series. Many uses have been assigned to them, as the holding of food, as they have been seen to hold to the fronds of seaweed and keep them steady until the spines and tube feet can be brought into action. The inner surface of the pedicellariæ are known to be the most sensitive, and the blades close on the minutest object touching the inner surface. Beside these peculiar bodies the surface of the skin has small tubular processes, and tubular feet with suckers at the end. At the extremity of each arm is a single tube-foot with an impaired tentacle, and above this again is a small eye coloured by red pigment.