LARVAL FORMS. 371 



3. The Trochosphere Group. This group (figs. 225, 226) 

 is characterised by the presence of a praeoral ring of long cilia, 

 the region in front of which forms a great part of the praeoral 

 lobe. The mouth opens immediately behind the praeoral ring 

 of cilia, and there is very often a second ring of short cilia 

 parallel to the main ring, immediately behind the mouth. The 



B. 



FIG. 222. A. LARVA OF EURYLEPTA AURICULATA IMMEDIATELY AFTER 

 HATCHING. VIEWED FROM THE SIDE. (After Hallez.) m. mouth. 



B. MULLER'S TURBELLARIAN LARVA (PROBABLY THYSANOZOON). VIEWED 

 FROM THE VENTRAL SURFACE. (After Muller.) The ciliated band is represented by 

 the black line. m. mouth ; u.l. upper lip. 



function of the ring of short cilia is nutritive, in that its cilia are 

 employed in bringing food to the mouth ; while the function of 

 the main ring is locomotive. A perianal patch or ring of cilia is 

 often present (fig. 225 A), and in many forms intermediate rings 

 are developed between the praeoral and perianal rings. 



The praeoral lobe is usually the seat of a special thickening 

 of epiblast, which gives rise to the supra-cesophageal ganglion of 

 the adult. On this lobe optic organs are very often developed 

 in connection with the supra-oesophageal ganglion, and a con- 

 tractile band frequently passes from this region to the oesophagus. 



The alimentary tract is formed of the three typical divisions. 



The body cavity is not developed directly as an outgrowth 

 of the alimentary tract, though the process by which it originates 

 is very probably secondarily modified from a pair of alimentary 

 outgrowths. 



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