LARVAL FORMS : THEIR NATURE, ORIGIN AND AFFINITIES. 391 
the existence of ciliated bands may be so also; but it is 
more probable that if, as I suppose, these larvae reproduce 
the characters of some ancestral form, this form may have 
existed at a time when all marine animals were free-swim- 
ming, and that it may, therefore, have been provided with 
at least one ciliated band. 
The detailed consideration of the characters of these 
larvae, given below, supports this view. 
This great class of larvae may, as already stated, be divided 
into a series of minor subdivisions. These subdivisions are 
the following : 
1. The Pilidium Group. — This group is characterised 
by the mouth being situated nearly in the centre of the 
ventral surface, and by the absence of a proctodaeum. It 
includes the Pilidium of the Nemertines (fig. 8), and the 
various larvae of marine Dendrocoela (fig. 9) . At the apex of 
the praeoral lobe a thickening of epiblast may be present, 
from which (fig. 19) a contractile cord sometimes passes to 
the oesophagus. 
Fig S . — Ttoo Stages in the development of Pilidium. (After Metsclmikoff.) 
ae. arclienteron ; oe. cesophagus ; st. stomach ; am. amnion ; pr.d. pro- 
stomial disc ; po.d. metastomial disc ; c.s. cephalic sack. 
2 . The Echinoderm Group. — This group(figs. 10, 11 , and 
18 c) is characterised by the presence of a longitudinal postoral 
band of cilia, the absence of special sense organs in the praeoral 
region, and the development of the body-cavity as an out- 
growth of the alimentary tract. The three typical divisions 
of the alimentary tract are present, and there is a more 
