STAR-FISHES 461 



E. Organs of Locomotion. 



If we watch a star-fish in the act of creeping, we notice a number of 

 white threads protruding under the arms, making it appear as if worms 

 were crawling out of its interior. These are processes of the integument, 

 the so-called sucking-feet, or pedicels, of which the animal possesses 

 hundreds, as a glance at its under side at once reveals. They are 

 placed close to each other in two double rows, along grooves extending 

 from the mouth (also placed on the ventral surface), to the tips of the 

 arms. While in repose they appear like small cutaneous papillae ; but 

 when the animal wishes to move they all extend in length, and turn 

 towards the side in which motion is to take place. They next attach 

 themselves to the supporting surface by their free ends, each of which 

 forms a small sucker (see suckers of cuttle-fish), after which they 

 contract, so that the body is drawn forwards by as many ropes, as 

 it were. Finally they once more become detached from the surface 

 of support, and the performance is repeated. By the help of these 

 suckers the star-fish is even able to climb up perpendicular walls 

 (e.g., the glass sides of the aquarium), and they also assist the animal in 

 the consumption of its prey (see Section G). These peculiar pedicels are 

 in their turn set in action by means of a 



F. Water -vascular System, 



such as we do not find repeated throughout the whole realm of Nature. 

 On the upper surface of the disc, near its centre, is seen a small cal- 

 careous plate, perforated like a sieve, and known as the madreporic plate, 

 through which sea-water enters from without into the body of the 

 animal. It is connected by a small canal called the stone canal, because 

 its walls are calcified, to another canal, which forms a ring around the 

 stomach near the mouth, and is hence called the circular canal. From 

 the latter a branch, the longitudinal canal, extends along each arm to its 

 tip, from which, again, lateral canals pass off like barbs from the shaft of 

 a feather. Each of these small lateral branches dilates inwardly into a 

 small vesicle (ampulla), while outwardly it is continued into the cavity 

 of the .tubular pedicels, with which we have already become acquainted. 

 When the muscles in the walls of these ampullae contract, water is 

 pumped into the tubular pedicels, which are consequently stretched in 

 length like a worm. On the other hand, if the muscles in the walls 

 of the foot contract, causing it to shorten, the water returns to the 

 corresponding ampulla. Thus, the star-fish can force water into all or 

 many or only a few of its pedicels, according to what is required for 

 the performance of any particular task. 



