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F. M. BALFOUR. 
or less developed prseoral lobe. This group only includes 
the larvae of the Echinodermata. 
3. The Trochosphere Group. — This group (figs. 12, 13) 
is characterised by the presence of a praeoral ring of long 
cilia, the region in front of which forms a great part of the 
prseoral 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 function of the ring of short cilia is nutritive, 
in that the cilia are employed in bringing food to the mouth ; 
while the function of the main ring is locomotive. A peri- 
anal patch or ring of cilia is often present (fig. 12 a), and in 
many forms intermediate rings are developed between the 
prseoral and perianal rings. 
The prseoral lobe is usually the seat of a special thicken- 
ing of epiblast, which gives rise to the supra-oesophageal 
A. 
Fig. 9. — A. Larva of Eur^lepia auriculata immediately after hatching. 
Viewed from the side. (After Hallez.) m. mouth. B. Muller's Tur- 
bellarian Larva {probably Thysanozoon). Viewed from the ventral 
surface. (After Miiller.) The ciliated baud is represented by the 
black line. m. mouth ; u.l. upper lip. 
ganglion of the adult. On this lobe optic organs are very 
often developed in connection with the supra-oesophageal 
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 does not develop 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. 
