THE NEW EVOLUTION 



greatly shorten the larval stages, or omit them 

 altogether. 



It is a curious fact that very closely related creatures 

 may have v^idely different larvas. For instance, the 

 larvas of nearly all the brittle-stars (fig. 44, p. 87) are 

 strange looking things called plutei (cf . fig. 89, p. 175), 

 but some brittle-stars have worm-like pelagic larvas. 

 The larvas of most starfishes (fig. 42., p. 71) are of the 

 form called bipinnarias, later usually becoming brachi- 

 olarias (fig. 88, p. 175); but starfishes w^hich brood their 

 young have w^orm-like larvas. 



On the other hand, very different creatures may 

 have very similar larvae. Thus the young of most, 

 but by no means all, of the mollusks and also of the 

 jointed w^orms or annelids are typical trochosphere 

 larvae. One adult rotifer has the same form. Larval 

 forms v^^hich are commonly considered modified tro- 

 chospheres are found in the echinoderms (fig. 109, 

 p. 175), in the balanoglossids, in the phoronids (fig. 

 lOi, p. 175), and in the polyzoans (fig. 90, p. 175). 



Extraordinary diversification of the larval forms is 

 found in the echinoderms. Here we have the barrel- 

 shaped larvas of the feather-stars (fig. 109, p. 175), the 

 auricularias of most of the holothurians or sea-cucum- 

 bers, which resemble most nearly the so-called tor- 

 naria larvae (fig. 94, p. 175) of the balanoglossids, the 

 bipinnarias and usually later the brachiolarias (fig. 88, 

 p. 175) of most starfishes, the bizarre plutei of most 

 sea-urchins (fig. 89, p. 175) and brittle-stars, and also 

 some other larval forms. 



Other curious larval forms among the marine ani- 



