Fig. 220. Larva of Argiope. (From Gegenbaur; after Kowalevsky.)
m. mantle; b. setæ; d. archenteron.
The detailed consideration of the characters of these larvæ, given below, supports this view.
This great class of larvæ 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 of the Nemertines ([fig. 221]), and the various larvæ of marine Dendrocœla ([fig. 222]). At the apex of the præoral lobe a thickening of epiblast may be present, from which ([fig. 232]) a contractile cord sometimes passes to the œsophagus.
Fig. 221. Two stages in the development of Pilidium.(After Metschnikoff.)
ae. archenteron; oe. œsophagus; 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.