LARVAL FORMS. 



311 



mata, Sagitta, etc. If these two points are granted, the further 

 conclusions seem to follow (1) that the ganglion and sense organs of 

 the prreoral lobe were secondary struc- 

 tures, which arose (perhaps as differentia- 

 tions of an original circular nerve ring) 

 after the assumption of a bilateral form ; 

 and (2) that the absence of these organs 

 in the larvae of the Echinodermata and 

 Actinotrocha (?) implies that these larvae 

 retain, so far, more primitive characters 

 than the Pilidium. The same may be 

 said of the alimentary cliverticula. There 

 are thus indications that in two important 

 points the Echinoderm larvse are more 

 primitive than the Pilidium. 



The above conclusions with reference 

 to the Pilidium and Echinoderm larvas in- 

 volve some not inconsiderable difficulties, 

 and suggest certain points for further dis- 

 cussion. 



In the first place it is to be noted 

 that the above speculations render it 

 probable that the type of nervous system from which that found 

 in the adults of the Echinodormata, Platyelminthes, Chsetopoda, 

 Mollusca, etc., is derived, was a circumoral ring, like that of Medusae, 



FIG. 230. ACTINOTROCHA. 

 (After Metschnikoff. ) 



m. mouth; an. anus. 



arn 



.7TZ 



FIG. 231. THREE DIAGRAMS REPRESENTING THE IDEAL EVOLUTION OF VARIOUS 



LARVAL FORMS. 



A. Ideal ancestral larval form. 



B. Larval form from which the Trochosphere larva may have been derived. 



C. Larval form from which the typical Echiuoderm larva may have been derived. 



m. mouth; an. anus; sf. stomach; s.fi. supra-oesophageal ganglion. 

 The black lines represent the ciliated bands. 



