STAR-FISHES. 



3°5 



vent and madreporito are on the upper side of the body. So little is known of the 

 Palaeozoic star-fishes and their relations to later forms, that all classifications must 

 be regarded as provisional. Subdivisions have been based on the character of the 

 so-called pedicellariae. These are small pincer-like organs that occur in star-fishes 

 and sea-urchins, on the surface of the test, as shown in the illustration below. 

 The movable spines covering the surface of these animals, and varying in size from 

 minute, delicate, bristle-like structures to long rods, which may be thin and 

 pointed, or thick, or even globular, are familiar to all. The pedicellariae are 

 probably derived from the smaller spines; two 

 of these united at the base by a muscle, and 

 slightly curved so as to approach one another 

 at the ends, form the simplest kind of pedicellaria ; 

 and, by gradual modifications of this type, all 

 the varieties may be derived. Many uses have 

 been ascribed to the pedicellariae, such as holding 

 pieces of food, or removing dirt from the surface 

 of the test. In some sea-urchins they are pro- 

 vided with poison-glands, which seems to show 

 that they serve as weapons of offence in those 

 cases. It has been considered that in sea-urchins 

 their chief use is to catch hold of fronds of 

 seaweed and keep them steady until the spines 

 and tube-feet can be brought into action. The 

 inner surfaces of the forceps in the pedicellariae 

 are remarkably sensitive, and the blades close on 

 any minute object immediately their inner sur- 

 faces are touched by it. It seems, however, that in spite of the amount of attention 

 devoted to these organs, we do not yet understand all their uses. Besides spines 

 and pedicellariae, star- fish also have on the surface of the skin small tubular 

 processes, containing an extension of the body-cavity. These have very thin, con- 

 tractile walls, and doubtless serve to assist respiration. All star-fishes have tube- 

 feet, but in some these have no suckers at the end, and in all cases those which are 

 at the ends of the rays are used only as feelers, and are stretched in the direction 

 in which the animal is moving. At the extremity of each arm is a single tube- 

 foot, which is the first to be formed, and is known as the unpaired tentacle ; this 

 being always stretched straight out. Immediately above this tentacle is a small 

 eye, coloured by red pigment, and protected by small tentacles. 



Star-fish are sluggish animals, rarely moving of themselves, and staying for 

 days in the same position. Those kept in tanks or in glass vessels prefer to cling 

 to the side, instead of lying on the bottom. When disturbed, however, a star-fish 

 can travel at a considerable pace. Those star-fish that have suckers crawl by means 

 of their tube-feet, while those that have no suckers still use their tube-feet slightly, 

 but also progress by the muscular movements of the rays. The short-armed 

 Asterina and Astropecten can right themselves in less than a minute, and accom- 

 plish the act by raising themselves on the tips of four rays, and then turning a 

 somersault by throwing over the fifth ray. Asterias takes rather longer, and 



vol. vi. — 20 



PEDICELLARIA. 



a, Two-jawed, closed ; b, Two-jawed, open ; 

 c, Three-jawed. (20 times nat. size. ) 



