HABITS, ETC., OF OEREBRATULUS LACTEUS. 157 



There have been several attempts to arrive at an idea of 

 their ancestral form, bat the way is beset by so many difficul- 

 ties that the attempts agree in few particulars. These few, 

 however, are significaut, for it is conceded that the archetype 

 must have been very simple, that it possessed radial symmetry, 

 and probably approached closely to a uniformly ciliated gas- 

 trula, with a rounded aboral surface and a flattened oral 

 surface, in the centre of which was situated the mouth. 

 Around the edge of the oral surface was a ciliated ring, pos- 

 sibly innervated. 



Even in these meagre details we recognise almost an exact 

 description of our Cerebratulus larva, which is, like that of 

 many Nemerteans, developing through a pilidium. 



The only noteworthy difference is the presence in the 

 pilidium of the apical plate with its bunch of flagella, which 

 forms a well-developed sense-organ. But in several impor- 

 tant respects the pilidium larva is less highly developed 

 than any of the others mentioned. This is especially seen in 

 the form of the alimentary canal, in the absence of an anus, 

 and in the lack of any true nervous system. In these par- 

 ticulars it approaches the ancestral type more closely than 

 any existing larva, and in consequence the manner in which 

 it changes from radial to bilateral symmetry becomes of 

 interest. 



The flattening of the inferior pole and the establishment 

 of the gastrula invagination leads to the fixing of a 

 structural axis passing vertically through the month, with 

 the whole body arranged radially around it. But the next 

 step is not an elongation of the aboral surface and the for- 

 mation of an anus there, as assumed for the ancestral 

 metamorphosis. 



It is rather an unequal elongation of the oral surface, 

 whereby the portion in front of the mouth forms the pre-oral 

 lobe, while that behind the mouth develops into the trunk 

 and forms an anus. 



Thus the aboral surface retains its relative position 

 and becomes the dorsal surface of the Nemertean. 



