LARVAL FORMS .* THEIR NATURE, ORIGIN AND AFFINITIES. 401 
Fig. 19. — A. Pilidium with an advanced Nemertine Worm. B. Ripe Embryo 
of Kemcrtes in the position it occupies in Pilidium. (Both after 
liutschli.) ce. oesophagus ; st. stomach ; i. intestine ; pr. proboscis ; 
Lp. lateral pit ; an. amnion ; n. nervous system. 
common origin. If this view is the correct one, we must 
suppose that the apex of the aboral lobe has become the 
centre of the prieoral field of the Pilidium and Trochosphere 
larval forms. ^ The whole of the questions concerning the 
nervous system are still very obscure, and until further facts 
are brought to light no definite conclusions can be arrived at. 
The absence of sense organs on the praeoral lobe of larval 
Echinoderrnata, coupled with the structure of the nervous 
‘ The independent development of the supra-cesopbageal ganglion and 
ventral nerve cord in Chaetopoda Kleinenberg, * Development of 
bricus trapezoid^s ’) suits this view very satisfactorily. 
closely with that of the auditory organ in Ctenophora ; and 
it is not impossible that the two structures may have had a 
