XVIII ECHINUS WATER-VASCULAR SYSTEM 5 I 7 



usual into the ring-canal, and is accompanied by the axial sinus 

 and genital stolon. The name " stone-canal " is very unsuitable 

 in this order, for there are no calcifications in its walls ; it is a 

 simple membranous tube of circular section. On reaching the 

 upper wall of the test it expands into an ampulla, into which 

 the numerous ciliated pore-canals traversing the madreporite 

 open. The radial canals, starting from the ring-canal, pursue a 

 downward course till they come into contact with the radial 

 nerve-cords, and they then bend upwards and run along the 

 centre of the ambulacral region, finally terminating in the small 

 terminal tentacles. In the just metamorphosed Echinoid these 

 are well-developed tube-feet, each with a well-developed sucker, 

 in the centre of which is a conical sensory prominence, but as 

 development proceeds they become enclosed in a circular out- 

 growth of the test, so that only the tip projects in the adult. 



The long extensible tube-feet are connected by transverse 

 canals with the radial canal. Instead of the pair of valves 

 which in Asteroids prevent the reflux of liquid into the canal, 

 there is a perforated diaphragm ^ with circular muscles, which 

 by contraction close the opening in the diaphragm, while 

 when they are relaxed fluid can return from the tube -foot. 

 The ampulla is flattened, and is contracted by muscular fibres 

 called " trabeculae " stretching across its cavity. These mus- 

 cular strands are developed by the cells lining the ampulla. 

 The external portion of the tube-foot, as in Asteroids, is pro- 

 vided with powerful longitudinal muscles, and there is the same 

 alternate filling and emptying of the ampulla as the tube-foot 

 is contracted and expanded. The tube-foot is connected by a 

 double canal with the ampulla, the object of which is to assist in 

 respiration. The cells lining it are ciliated, and produce a 

 current up one side of the tube-foot and down the other, and 

 the double canal leading to the ampulla separates these two 

 currents and prevents them interfering with one another. Thus 

 water is continually transported from the ampulla to the tube- 

 foot, through the thin walls of which it absorbs oxygen, and it is 

 then carried back to the ampulla, and transfers its oxygen to the 

 fluid of the general body-cavity through the walls of the ampulla. 

 The disc of the tube-foot is supported by a calcareous plate 



1 Cuenot, "fitudes Morphologiques sur les ^fichinodermes," .^rcA. Biol. xi. 1891, 

 p. 544. 



