KCHINO DERMA TA . 



469 



into a series of 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 re- 

 main 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 postanal 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 : 



(1) The presence of the post-anal 

 lobe at the hind end of the body. 

 (2) The slight development of a prae- 

 oral lobe. (3) The provisional calca- 

 reous skeleton in the larval arms. 



Great variations are presented in 

 the development of the arms and pro- 

 visional skeleton. The presence of 

 lateral arms is however a distinctive 

 characteristic of the Ophiuroid Plu- 

 teus. The other arms may be quite 

 absent, but the lateral arms never. 



The formation of the permanent 

 Ophiuroid takes place in much the 

 same way as in the Aster oidea. 



There is formed (fig. 262) on the 

 and dorsal side of stomach the 

 abactinal disc supported by calcareous 

 plates, at first only five in number and radial in position 1 . The disc is at 

 first not symmetrical, but becomes so at the time of the resorption of the 

 larval arms. It grows out into five processes the five future rays. The 

 original five radial plates remain as the terminal segments of the adult 



right 



FlG. 262. PjjUTEDS LARVA OF AN 



OPHIUROID. (From Gegeubaur; after 

 Miiller.) 



A. rudiment of young Ophiuroid; 

 d'. lateral arms; d. anterior arms; 

 e'. posterior arms. 



1 Whether interradial plates are developed as in Asterias is not clear. They seem 

 to be found in Ophiopholis bellis, Agassiz, but have not been recognised in other forms 

 (vide Carpenter, No. 548, p. 369). 



