XIV] ASTERIAS 333 



swing which is parallel to the same line in all the arms, not neces- 

 sarily parallel to the axis of the arm to which the tube-feet belong. 

 We found that in order to examine the alimentary canal it was 

 advisable to divide the star-fish into an upper and a lower half. If 

 we now cut away the tube-feet and look at the roof of the ambu- 

 lacral groove from which they project, it will be seen that the 

 groove is roofed in by a double series of calcareous rods, meeting 

 each other at an angle like the beams of a church-roof (1 1, Fig. 153). 

 These are called the ambulacral ossicles. They can be drawn 

 together by muscle fibres running from one of a pair to its fellow 

 just under the spot where they meet. By this action the ambu- 

 lacral groove is narrowed ; and at the same time, inwardly projecting 

 spines lining its edges are made to meet, so that the tube-feet are 

 entirely protected by a trelliswork of spines. The spines are attached 

 to rods, called the adambulacral ossicles, which are firmly 

 bound to the outer edges of the ambulacral ossicles (12, Fig. 153). 

 Inside the animal, between the ambulacral plates, a series of pear- 

 shaped transparent bladders tensely filled with fluid project into 

 the coelom (Figs. 151 and 153). These are really the swollen upper 

 ends of the tube-feet and are termed ampullae. They act as 

 reservoirs into which the fluid contents of the lower part of the foot 

 are driven when the longitudinal muscles of the tube-foot contract. 

 The bladder-like upper end of the foot has only circular muscles, 

 and when these contract the fluid is driven back into the lower part 

 of the tube-foot and this is thus expanded. The tube-feet are really 

 all parts of one system, though from the above description it would 

 seem as if each was capable of acting without the others : but they 

 are connected by short transverse tubes, with a canal running 'along 

 the whole length of the arm immediately under the ambulacral 

 ossicles, called the radial water-vessel (13, Fig. 153). This 

 canal and its branches can easily be seen in microscopic sections of 

 the arms of young star-fish, or they can readily be demonstrated 

 by cutting off the tip of the arm of a fully-grown specimen, 

 finding the end of the radial tube on the cut surface and inject- 

 ing it with coloured fluid by means of a fine pipette. The five 

 radial tubes are connected with each other by a ring-shaped 

 canal lying just within the peristome, which is called the water- 

 vascular ring. There are nine small pouches called Tie de- 

 man n's bodies projecting inwards from the ring canal. In these 

 are formed the amoebocytes which are found floating in the fluid of 

 the canal, and which arise by budding from the wall of each pouch. 



