306 



THE TROCHOSPHERE GROUP. 



tines (fig. 221), and the various larva? of marine Dendroccela (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. Th is 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 prseoral 

 region, and by the development of the body-cavity as an outgrowth 

 of the alimentary tract. The three typical divisions of the alimen- 

 tary tract are present, and there is a more or less developed praeoral 

 lobe. This group only includes the larvie of the Echinodermata. 



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 



B. 



FIG. 222. A. LARVA OF EUKYLEPTA AUKICULATA IMMEDIATELY AFTER HATCHING. 

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



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

 VENTRAL SURFACE. (After Miiller.) The ciliated hand is represented by the black line. 

 in. mouth; u.l. upper lip. 



is very often a second ring of short cilia parallel to the main ring, 

 immediately behind the mouth. The 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 prseoral 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 con- 

 nection with the supra-cesophageal ganglion, and a contractile band 

 frequently passes from this region to the oesophagus. 



