632 THE INVERTEBRATA 



Stronger and more numerous in a longitudinal band around the ventral 

 concavity. This band is the organ of locomotion. Growing more 

 rapidly than the rest of the ectoderm, it becomes thrown into folds, 

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

 echinoderms), whose length and arrangement differ so as to character- 

 ize a special type of larva in each class (Fig. 439). 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 



Fig. 439. 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. 

 Q, Bipinnaria. U, Pluteus. E, Crinoid larva, y^n. anus; M. mouth; /)r.' pre- 

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



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

 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 



