Fig. 189.—View of a Starfish (Echinaster) devouring a Mussel. 1. The madreporite.
The favourite food[[443]] of Asterias consists of the common bivalves of the coast, notably of the Mussel (Mytilus edulis). There is, however, no animal which it will not attack if it is fortunate enough to be able to catch it. The Starfish seizes its prey by the tube-feet, and places it directly under its mouth, folding its arms down over it in umbrella fashion. The muscles which run around the arms and disc in the body-wall contract, and the pressure thus brought to bear on the incompressible fluid contained in the coelom, forces out the thin membranous peristome and partially turns the stomach inside out. The everted edge of the stomach is wrapped round the prey.
Soon the bivalve is forced to relax its muscles and allow the valves to gape. The edge of the stomach is then inserted between the valves and applied directly to the soft parts of the prey which is thus completely digested. When the Starfish moves away nothing but the cleaned shell is left behind. If the bivalve is small it may be completely taken into the stomach, and the empty shell later rejected through the mouth.
It was for a long time a puzzle in what way the bivalve was forced to open. Schiemenz[[444]] has, however, shown that when the Starfish folds itself in umbrella-like form over the prey it holds on to the substratum by means of the tube-feet of the distal portions of the arms, whilst, by means of the tube-feet belonging to the central portions, it drags apart the valves by main force. He has shown experimentally: (1) that whilst a bivalve may be able to resist a sudden pull of 4000 grammes it will yield to a pull of 900 grammes long continued; (2) that a Starfish can exert a pull of 1350 grammes; (3) that a Starfish is unable to open a bivalve unless it be allowed to raise itself into a hump, so that the pull of the central tube-feet is at right angles to the prey. A Starfish confined between two glass plates walked about all day carrying with it a bivalve which it was unable to open.
The lining of the stomach is found to consist very largely of mucus-forming cells, which are swollen with large drops of mucus or some similar substance. It used to be supposed that this substance had some poisonous action on the prey and paralysed it, but the researches of Schiemenz show that this is incorrect. If when an Asterias is devouring a bivalve another be offered to it, it will open it, but will not digest it, and the victim shows no sign of injury but soon recovers. The cells forming the walls of the pyloric sac and its appendages are tall narrow cylindrical cells crowded with granules which appear to be of the nature of digestive ferment. This substance flows into the stomach and digests the captured prey.
A very small amount of matter passes into the rectum and escapes by the anus, as the digestive powers of the Starfish are very complete. The rectal caeca are lined by cells which secrete from the coelomic fluid a brown material, in all probability an excretion, which is got rid of by the anus.
When the meal is finished the stomach is restored to its former place by the action of five pairs of retractor muscles, one pair of which originates from the upper surface of the ambulacral ossicles in each arm and extends to the wall of the stomach, where they are inserted (Fig. 190, ret).
The tube-feet, which are at once the locomotor and the principal sensory organs of the Starfish, are appendages of that peculiar system of tubes known as the water-vascular system, which is derived from a part of the coelom cut off from the rest during the development of the animal. This system, as already mentioned, consists of (1) a narrow "ring-canal," encircling the mouth and lying on the inner surface of the membranous peristome; (2) a radial canal leaving the ring-canal and running along the under surface of each arm just above the ambulacral groove; (3) a vertical stone-canal running from the madreporite downwards to open into the ring-canal in the interspace between two arms. The madreporite is covered externally by grooves lined with long cilia, and is pierced with narrow canals of excessively fine calibre, the walls of which are also lined by powerful cilia. Most of these narrow canals open below into a main collecting canal, the stone-canal, but some open into a division of the coelom termed the axial sinus, with which also the stone-canal communicates by a lateral opening. The cavity of the stone-canal is reduced by the outgrowth from its walls of a peculiar Y-shaped projection, the ends being rolled on themselves in a complicated way (Fig. 190, B). The walls of the canal consist of a layer of very long narrow cells, which carry powerful flagella, and outside this of a crust of calcareous deposit, which gives rigidity to the walls and has suggested the name stone-canal.
The tube-feet are covered externally by ectoderm, inside which is a tube in connexion with the radial water-vascular canal. This latter is lined by flattened cells, which in the very young Starfish are prolonged into muscular tails; in the older animal these tails are separated off as a distinct muscular layer lying between the ectoderm and the cells lining the cavity of the tube. The tube-foot is prolonged inwards into a bulb termed the "ampulla," which projects into the coelom of the arm and in consequence is covered outside by somatic peritoneum. Just where the ampulla passes into the tube-foot proper the organ passes downwards between two of the powerful ambulacral ossicles which support the ambulacral groove, and a little below this spot a short transverse canal connects the tube-foot with the radial canal which lies beneath these ossicles (Fig. 191).