PRyEORAL LOBE. 269 



that of Other Echinoderm larvae, is the well-developed 

 apical plate at its anterior extremity. We may express 

 this in other words by saying that the larva of Antedon 

 possesses a central nervous system at the apex of its 

 praeoral lobe. That the pras- 

 oral lobe in this larva is not 

 sharply marked off from the 

 rest of the body is a detail 

 of no morphological signifi- 

 cance. 



The apical nervous sys- 

 tem of the Antedon larva 

 was discovered in 1888 by 



H. Bury, and has been Fig. 128. — Free-swimming larva of 



111 1 . . Antedon rosacea, from the ventral side. 



more clearly brought out (After seeliger.) 



and emphasised in a recent "-t- Epical pole. c.b. ciliated bands. 



f. Fixing disc. v. Vestibulum (so-called 

 work by Dr. Oswald SeELI- iarval mouth, although at this stage 

 A . . 1 • . 1 • 1 ■ simply an ectodermic groove). 



GER. At the point which is '^■' & > 



marked externally by the anterior tuft of long cilia in 

 Fig. 129 there is a slight groove in the ectoderm below 

 which nerve-fibres and ganglion-cells can be identified. 

 Seeliger further describes a pair of longitudinal nerves 

 running from the nervous area of the apex along the 

 ventro-lateral margins of the body. 



As already indicated, the apical plate is, as a general 

 rule, conspicuous by its absence in the typical Echinoderm 

 larva. In the free-swimming larva of Antedon, however, 

 it is emphatically present, although destined to become 

 entirely aborted after the fixation of the larva. 



In most Invertebrate larvae in which an apical plate is 

 present {e.g. the Trochophore-larva of Annelids and Mol- 

 luscs) it becomes, during the metamorphosis, involved in 

 other ectodermic thickenings of the pra;oral lobe, which 



