4. Tornaria.--This larva (fig. 229) is intermediate in most of its characters between the larvæ of the Echinodermata (more especially the Bipinnaria) and the Trochosphere. It resembles Echinoderm larvæ in the possession of a longitudinal ciliated band (divided into a præoral and a postoral ring), and in the derivation of the body cavity and water-vascular vesicle from alimentary diverticula; and it resembles the Trochosphere in the presence of sense organs on the præoral lobe, in the existence of a perianal ring of cilia, and in the possession of a contractile band passing from the præoral lobe to the oesophagus. [FIG. 224. A LARVA OF STRONGYLOCENTRUS. (From Agassiz.) _m._ mouth; _a._ anus; _o._ oesophagus; _d._ stomach; _c._ intestine; _['v]._ and _v._ ciliated ridges; _w._ water-vascular tube; _r._ calcareous rods.] 5. Actinotrocha.--The remarkable larva of Phoronis (fig. 230), known as Actinotrocha, is characterised by the presence of (1) a postoral and somewhat longitudinal ciliated ring produced into tentacles, and (2) a perianal ring. It is provided with a præoral lobe, and a terminal or somewhat dorsal anus. 6. The larva of the Brachiopoda articulata (fig. 220). The relationships of the six types of larval forms thus briefly characterised have been the subject of a considerable amount of controversy, and the following suggestions on their affinities must be viewed as somewhat speculative. The Pilidium type of larva is in some important respects less highly differentiated than the larvæ of the five other groups. It is, in the first place, without an anus; and there are no grounds for supposing that the anus has become lost by retrogressive changes. If for the moment it is granted that the Pilidium larva represents more nearly than the larvæ of the other groups the ancestral type of larva, what characters are we led to assign to the ancestral form which this larva repeats? [FIG. 225. TWO CHÆTOPOD LARVÆ. (From Gegenbaur.) _o._ mouth; _i._ intestine; _a._ anus; _v._ præoral ciliated band; _w._ perianal ciliated band.] In the first place, this ancestral form, of which fig. 231 A is an ideal representation, would appear to have had a dome-shaped body, with a flattened oral surface and a rounded aboral surface. Its symmetry was radial, and in the centre of the flattened oral surface was placed the mouth, and round its edge was a ring of cilia. The passage of a Pilidium-like larva into the vermiform bilateral Platyelminth form, and therefore it may be presumed of the ancestral form which this larva repeats, is effected by the larva becoming more elongated, and by the region between the mouth and one end of the body becoming the præoral region, and by an outgrowth between the mouth and the opposite end developing into the trunk, an anus becoming placed at its extremity in the higher forms.