_ae._ archenteron; _oe._ oesophagus; _st._ stomach; _am._ amnion; _pr.d._ prostomial disc; _po.d._ metastomial disc; _c.s._ cephalic sack (lateral pit).] 2. The Echinoderm Group.--This group (figs. 223, 224 and 231 C) is characterised by the presence of a longitudinal _postoral_ band of cilia, by the absence of special sense organs in the præoral 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 præoral lobe. This group only includes the larvæ of the Echinodermata. 3. The Trochosphere Group.--This group (figs. 225, 226) is characterised by the presence of a præoral ring of long cilia, the region in front of which forms a great part of the præoral lobe. The mouth opens immediately behind the præoral ring of cilia, and there 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 præoral and perianal rings. [FIG. 222. A. LARVA OF EURYLEPTA AURICULATA IMMEDIATELY AFTER HATCHING. VIEWED FROM THE SIDE. (After Hallez.) _m._ mouth. B. MÜLLER'S TURBELLARIAN LARVA (PROBABLY THYSANOZOON). VIEWED FROM THE VENTRAL SURFACE. (After Müller.) The ciliated band is represented by the black line. _m._ mouth; _u.l._ upper lip.] The præoral lobe is usually the seat of a special thickening of epiblast, which gives rise to the supraoesophageal ganglion of the adult. On this lobe optic organs are very often developed in connection with the supraoesophageal ganglion, and a contractile 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. Paired excretory organs, opening to the exterior and into the body cavity, are often present (fig. 226 _nph_). This type of larva is found in the Rotifera (fig. 217) (in which it is preserved in the adult state), the Chætopoda (figs. 225 and 226), the Mollusca (fig. 218), the Gephyrea nuda (fig. 227), and the Polyzoa (fig. 228)[140]. [140] For a discussion as to the structure of the Polyzoon larva, _vide_ Vol. II. p. 305. [FIG. 223. A. THE LARVA OF A HOLOTHUROID. B. THE LARVA OF AN ASTEROID. _m._ mouth; _st._ stomach; _a._ anus; _l.c._ primitive longitudinal ciliated band; _pr.c._ præoral ciliated band.]