332 ECHINODERMATA [CH. 



appendages, which flows downwards into the stomach. Any portion 

 remaining undigested is expelled through the rectum ; no food ever 

 penetrates into the pyloric caeca. 



The locomotion of the star-fish is effected in the following 



manner. The tube-feet which crowd the ainbulacral grooves are 



during life continually extended and retracted. At their ends are 



water-vas- fl a ^ circular discs, and these discs are pushed against 



cuiar system, t ke stone or roc ^ or wna tever else the star-fish is 



i 5 



9 . yjgSh-- J__ '' ' ' MM 



JO 



FIG. 153. Diagram of a transverse section of the arm of a Star-fish. 



1. Ectoderm. 2. Jelly. 3. Peribranchial space in the skin. 



4. Peritoneal lining of body cavity. 5. A branchia. 6. Pyloric 



caecum. 7. Mesentery supporting a caecum. 8. Spine. 9. Ossicle 

 in skin. 10. Pedicellaria. 11. Ambulacral ossicle. 12. Adambu- 

 lacral ossicle. 13. Kadial trunk of water-vascular system. 



14. Radial septum separating the two perihaemal spaces. 15. Eadial 



nerve-cord, a thickened band of ectoderm with a plexus of nerve-fibrils 

 underlying it. 16. Ampulla of tube-foot. 17. Tube-foot. 



18. Perihaemal space. 19. Coelom. 



clinging to. Then by the contraction of their muscles the centre 

 of the disc is pulled upwards, and so it is made to adhere in exactly 

 the same way in which a boy makes a leather "sucker" adhere to a 

 stone. When once the disc is firmly fixed the contraction of the 

 tube-foot draws the animal after it. This description applies to the 

 movements of the star-fish when it is climbing, but when it is 

 walking over level ground the tube-feet are used just like the legs 

 of a man in walking, i.e. they push against the ground, they are 

 then withdrawn, swing forward, and push again. The direction in 

 which the star-fish moves is determined by the direction of the 



