566 ECHINOID PLUTEUS. 



Pluteus, which accordingly has, as a rule, a much narrower form 

 than the Ophiuroid Pluteus. 



A pair of ciliated epaulettes on each side of and behind the 

 ciliated ring is very characteristic of some Echinoid larvae. 

 They are originally developed from the ciliated ring (fig. 266 A 



FIG. 265. LATERAL AND VENTRAL VIEW OF A LARVA OF STRONGYLOCENTRUS. 

 (From Agassiz.) General references as in fig. 264. 



b. dorsal opening of madreporic canal; e . posterior arms; e'". anterior arms; 

 e' v . anterointernal arms. 



and B, ?>"). The presence of three processes from the anal lobe 

 supported by calcareous rods is characteristic of the Spatangoid 

 Pluteus (fig. 263 E). 



The first two pairs of arms to develop, employing the same names as in 

 Ophiuroids, are the anterior attached to the oral process (fig. 263 C, d} and 

 the posterior pair (e'). A pair of anterolateral arms next becomes developed 

 (ft). A fourth pair (not represented in Ophiuroids) appears on the inner 

 side of the anterior pair forming an anterointernal pair (e), and in the 

 Spatangoid Pluteus a fifth pair may be added on the external side of the 

 anterior pair forming an anteroexternal pair (g). 



Each of the first-formed paired calcareous rods is composed of three 

 processes, two of which extend into the anterior and posterior arms ; and the 

 third and strongest passes into the anal lobe, and there meets its fellow 

 (fig. 265). A transverse bar in front of the arms joins the rods of the two 

 sides meeting them at the point where the three processes diverge. The 

 process in the anterolateral arm (fig. 266 B) is at first independent of this 

 system of rods, but eventually unites with it. Although our knowledge of 



