ECHINODERMATA. 



563 



processes along the ciliated band, which is continued to the 

 extremity of each. The primitive ciliated ring never becomes 

 broken up into two or more rings. A ciliated crown is usually 

 developed at the extremity of the post-anal lobe. The arms are 

 arranged in the form of a ring surrounding the mouth, and are 

 all directed forwards. 



The first arms to appear are two lateral ones, which usually remain the 

 most conspicuous (fig. 261 B and C, d'}. Next arises a pair on the sides of 

 the mouth, which may be called the mouth or anterior arms (C, d}. A pair 

 ventral to and behind the lateral arms is then formed, constituting the 

 posterior arms (D, e'\ and finally a pair between the lateral arms and the 

 anterior, constituting the anterolateral arms (D,^). 



The concave area between the arms forms the greater part of 

 the ventral surface of the body. Even before the appearance of 

 any of the arms, and before the formation of the mouth, two 

 calcareous rods are formed, which meet behind at the apex of 

 the post-anal lobe, and are continued as a central support into 

 each of the arms as they are successively formed. These rods 

 are shewn at their full development in fig. 262. The important 

 points which distinguish a Pluteus 

 larva from the Auricularia or 

 Bipinnaria are the following : 



(i) The presence of the post- 

 anal lobe at the hind end of the 

 body. (2) The slight develop- 

 ment of a prae-oral lobe. (3) The 

 provisional calcareous skeleton in 

 the larval arms. 



Great variations are presented 

 in the development of the arms 

 and provisional skeleton. The 

 presence of lateral arms is however 

 a distinctive characteristic of the 

 Ophiuroid Pluteus. The other 

 arms may be quite absent, but 

 the lateral arms never. 



The formation of the perma- 

 nent Ophiuroid takes place in 

 much the same way as in the Asteroidea. 



36-2 



FIG. 262. PLUTEUS LARVA OF AN 

 OPHIUROID. (From Gegenbaur ; 

 after Miiller.) 



A. rudiment of young Ophiuroid ; 

 d '. lateral arms; d. anterior arms; 

 e ' . posterior arms. 



