56 BRANCH ECHINODERMATA 



The nervous system consists of a circumoral nerve ring, 

 from which a nerve proceeds along the ambulacral groove of 

 each arm to its tip, where it ends in a so-called " eye-spot " 

 which has been proved sensitive to light. 



Special Senses. — Besides the general sense of touch and the 

 " eye-spots," already mentioned, there is at the distal end of 

 each ray a tentacle-like organ which is supposed to be the organ 

 of smell. 



Multiplication is sexual. Fertilization takes place in the 

 water. The starfish may reproduce asexually, for if a ray be 

 broken off,' either accidentally or purposely by the animal 

 itself, it has the power of reproducing a new disk as well as the 

 rest of the arms, with their internal organs. Similarly, if all 

 the arms are torn off, the disk has the power of growing out new 

 ones. The young are bilaterally symmetric, free-swimming 

 animals. The metamorphosis is complicated, resulting finally 

 in the rachal plan of structure of the adult. 



The starfish, Linckia linckia, is a host for a parasitic gastero- 

 pod (Thyca). Some starfishes are gregarious. 



In size they vary from less than 1 inch to 3 feet in diameter. 

 In color they may be >'ellow, brown, red, or purple. 



Geologic Distribution. — The starfishes appeared before the 

 close of the Cambrian Period, and have been represented in 

 every age up to the present. 



CLASS n. OPHIUROIDEA 



These echinoderms resemble the starfish. The arms are 

 slender, jointed, muscular, and are used for locomotion (Fig. 

 43). The arms may be much branched, as in the basket-fish, 

 and are not hollow as they are in the starfish. The ambulacral 

 groove is closed, the tube-feet are on the sides of the arms, 

 and have no suckers at their distal ends. 



The arms are much more slender and more flexible than those 

 of the starfish, and locomotion, which is faster than that of the 

 starfish, is accomplished l)y the lateral movements of the arms, 

 Some species have the power of throwing off pieces of their 

 arms when disturbed. 



The digestive organs are confined to the disk, the hepatic 

 1 Parker and HiiswcU, p. 400. 



