556 THE INVERTEBRATA 



the larval arms (which have nothing to do with the arms of adult echi- 

 noderms), whose length and arrangement differ so as to characterize 

 a special type of larva in each class (Fig. 414). In the Auricularia larva 

 of the Holothuroidea the body is elongate and the band lengthens 

 fore and aft and outlines a strong preoral lobe . The Bipinnaria of the 

 Asteroidea resembles the Auricularia in general features, but in it the 

 border of the preoral lobe separates completely from the rest of the 



Fig. 414. Diagrams of echinoderm larvae. The postoral part of the early 

 ciliated band is drawn heavily (except where remote), the preoral part cross- 

 hatched. A, Early stage with simple continuous band. B, Auricularia. 

 C, Bipinnaria. D, Pluteus. E, Crinoid larva. An. anus; M. mouth; pr.' 

 preoral band ; pr." corresponding part of continuous band ; pt. postoral band. 



longitudinal band. In the Plutei of the Ophiuroidea and Echinoidea 

 the band remains continuous, but forms only a small preoral lobe, and 

 the postanal region of the body develops greatly, while the slender 

 arms are supported by calcareous rods. The Pluteus of the Ophiuroidea 

 (Ophiopluteus) has a different appearance from that of the sea urchins 

 (Echinopluteus), owing to the fact that the former of these larvae has 

 fewer arms than the latter and that in it the arms known as the " pos- 

 terolateral arms" are the largest and are directed forwards, whereas 

 these arms, if they are present in the Echinopluteus , are there small 



