370 THE ECHINODERM GROUP. 



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 an anus. It includes the Pilidium 



FlG. 221. TWO STAGES IN THE DEVELOPMENT OF PlLIDIUM. 



(After Metschnikoff.) 



ae. archenteron; oe. oesophagus; st. stomach; am. amnion; pr.d. prostomial 

 disc ; pod. metastomial disc ; c.s. cephalic sack (lateral pit). 



of the Nemertines (fig. 221), and the various larvae of marine 

 Dendrocoela (fig. 222). At the apex of the praeoral lobe a 

 thickening of epiblast may be present, from which (fig. 232) a 

 contractile cord sometimes passes to the oesophagus. 



2. The Echinoderm Group. This group (figs. 223, 224 

 and 231 C) is characterised by the presence of a longitudinal 

 pastoral band of cilia, by the absence of special sense organs in 

 the praeoral region, and by the development of the body cavity 

 as an outgrowth of the alimentary tract. The three typical 

 divisions of the alimentary tract are present, and there is a more 

 or less developed praeoral lobe. This group only includes the 

 larvae of the Echinodermata. 



