362 



ECHINODERMATA 



[CH. 



reminds one of a star-fish, in the fact that on the oral sides of 

 these arms there are open grooves converging to the mouth, and 

 that the skin lining these grooves is modified to form nervous bands 

 uniting in a ring round the mouth. These ambulacra! grooves are 

 further lined by powerful cilia which cause currents of water carry- 

 ing small animals to flow towards the mouth, and thus the animal is 



FIG. 170. Transverse section through the disc and base of an arm of 

 Antedon rosacea. After Ludwig. 



1. Mouth. 2. Various sections of alimentary canal. 3. Epithelium of 

 ambulacral groove. 4. Nervous layer of ambulacral groove. 5. Eadial 

 water-canal. 6. Circumoral water-vascular ring. 7. Stone-canals. 



8. Pore-canals. 9. Trabeculae traversing the coelom. 10. Axial 



coelom communicating with 11. 11. Coelom of arm. 12. Cirri. 



13. Genital stolon giving off 14. 14. Branches of genital stolon in 



cirri. 15. Eadial nerve of dorsal nervous system. 16. "Chambered 

 organ" in centre of dorsal nervous system. 17. Calcareous rods 



developing into trabeculae. 18. Muscles connecting radial and brachial 

 plates. CD. Centro-dorsal piece. K 1 , R 2 , E 3 . First, second and third 

 radial plate. Br 1 , Br 2 . First and second brachial plate. 



fed. The tube-feet are small and apparently of use only as gills, 

 those springing from the grooves on the disc alone retaining their 

 sensory function. 



The skeleton is peculiar. The ventral side of the body is 

 covered by a leathery skin, but on the aboral side there are first the 

 centro-dorsal ossicle, a round knob representing the uppermost 



