LARVAL FORMS. 



377 



of larvae are provided with them. Alimentary diverticula are 

 characteristic of the larvae of the Echinodermata and of Tprnaria. 



If the conclusion already arrived at to the effect that the 

 prototype of the six larval groups was descended from a radiate 

 ancestor is correct, it appears to follow that the nervous system, 

 in so far as it was differentiated, had primitively a radiate form ; 

 and it is also probably true that there were alimentary diverticula 

 in the form of radial pouches, two of which may have given 

 origin to the paired diverticula which become the body cavity in 

 such types as the Echinodermata, Sagitta, etc. If these two 

 points are granted, the further conclusions seem to follow (i) 

 that the ganglion and sense organs of 

 the praeoral lobe were secondary struc- 

 tures, which arose (perhaps as diffe- 

 rentiations 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 diverticula. There are 

 thus indications that in two important 

 points the Echinoderm larvae are more 

 primitive than the Pilidium. 



The above conclusions with refer- 

 ence to the Pilidium and Echinoderm 

 larvae involve some not inconsiderable 

 difficulties, and suggest certain points for further discussion. 



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 Echinodermata, Platyelminthes, 

 Chaetopoda, Mollusca, etc., is derived, was a circumoral ring, 

 like that of Medusae, with which radially arranged sense organs 

 may have been connected ; and that in the Echinodermata this 

 form of nervous system has been retained, while in the other types 

 it has been modified. Its anterior part may have given rise to 

 supra-cesophageal ganglia and organs of vision ; these being 



FIG. 230. ACTINOTROCHA. 

 (After Metscbnikoff.) 



m. mouth ; an. anus. 



