16 AMONG THE SEA-URCHINS. 



the nmbulacral areas, which are connected with the tube feet. The 

 circular canal also supplies five si)ecial membranous sacs or 

 bladders, called the "Polian vesicles," which act as reservoirs to 

 receive the water when it is suddenly expelled from the tube feet. 

 Each tube foot is divided near its base, one portion passing 

 through one pore into the radiating canal, and thus on to the 

 circular canal round the gullet with its reservoirs, while the other 

 portion passes through the other pore into a small contractile 

 vesicle, which, like the tube foot itself, is provided with rings of 

 muscular fibre. These tiny bladders, or " Ampullae," as they are 

 called, can be seen in our woodcut (Fig. 2, C, g-g), on the inner 

 side of the shell. When the muscles of these reservoirs contract, 

 the contained fluid is driven into the tube feet, which are then 

 protruded ; and similarly when these muscles relax, the pressure 

 is diminished, and the tube foot retracts. Similar muscular action 

 takes place on a greater scale with the large " Polian vesicles," 

 " a mechanism," says Professor Romanes, " of special interest, as 

 being unique in the animal kin'gdom, for the animal is thus 

 furnished with the means of varying the head of pressure in its 

 tube feet, either locally or universally." 



We were most interested in the evenings during our visit to 

 Port Erin with our investigations into the structure of the tube 

 feet, and the singular way in w^hich the muscular fibre is strength- 

 ened by the addition of curious calcareous spicules within them. 

 Our sponge investigations had made us very familiar with 

 various forms of calcareous and siliceous sponge spicules, but we 

 had never previously heard of these forms, and possibly many of 

 our readers may hear of them now for the first time. 



The beautiful calcareous rosette which forms the suctorial disc 

 at the end of each tube foot was well known (see Plate I., Fig. 2), 

 having been fully described by the late P. H. Gosse in his 

 " Evenings at the Microscope " :— " A glassy plate of extreme 

 thinness, circular in form, lies free in the swollen cavity of the 

 termination of the tube. It is cut into four or five incisions, 

 which reach almost to the centre, which is perforated with a large 

 round orifice." He then states that the marginal notching seen 

 on the edge is not real, but apparent only. In this he is in error, 

 for if the tube foot is entirely dissolved in caustic potash the rosette 



