164 THE SEA SHOEE 



alter the position of the centre of gravity till eventually the bodj 

 topples over to the desired position. 



Some of the common five-rayed stars have no suckers on their 

 tube-feet, and consequently have to creep by means of the muscular 

 contractions of their arms ; and several of them are like the brittle 

 stars in breaking up their bodies when irritated or seized. This latter 

 peculiarity will account for the frequency with which we come across 

 animals with one or more rays smaller than the others, the smaller 

 rays being new ones that have been produced in the place of those 

 lost. Again, we sometimes meet with such monstrosities as a five- 

 rayed star with six or more rays, some smaller than others, the 

 smaller ones representing two or more that have grown in the place 

 of one that has been lost ; or a starfish with branched or forked 

 arm, illustrating the tendency to produce a new arm even when the 

 original one has been only partially severed. 



A close observation of a starfish in water may enable us to 

 detect a number of little transparent processes standing out between 

 the prominences of the rough skin of the upper surface. These are 

 little bags filled with fluid, formed of such thin walls that gases can 

 readily pass through them, and are undoubtedly connected with the 

 process of respiration. Also, on the upturned extremity of each 

 arm a red spot may be seen ; and this from the nature of its struc- 

 ture, and from its association with the nervous system, has been 

 regarded as a rudimentary eye. 



On the upper side of the disc one may also observe a more or 

 less conspicuous spot of variable colour, on one side of the 

 centre. It is a plate, finely perforated, covering the outer extremity 

 of a short canal which communicates with the system of water tubes 

 that were described in the earlier part of this chapter. It is, in 

 fact, the entrance through which water is admitted into the central 

 ring round the mouth, and from this into the radial water tubes that 

 run through each arm of the starfish to supply the tube-feet. The 

 short tube referred to is always filled with sand, and thus the 

 water that enters into the water- vascular system is filtered before 

 it reaches the circular vessel. It is interesting to note, in this con- 

 nection, that here is one respect in which the radiate symmetry of 

 the starfish is broken, there being only one entrance, and that 

 not a central one, by which water is distributed into the five rays. 



Of course, when the ray of a starfish has been broken off 

 the water vessel or vessels that it contained are destroyed, as is 

 also the prolongation of the stomach, in the form of a long, blind 



