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The simplest of downstream larvae are Müller's larva (and the smaller Götte's larva) of polyclad flatworms and the pilidium of nemertines. These are topologically the same: both have an anteriorly positioned apical organ or apical tuft, a ventroposterior mouth leading to a blind gut with no anus, and a circumferential ciliary band or set of bands. Polyclad larvae are are generally compact, solid and opaque. The band runs along the margins of a set of projecting lobes, six or eight in number. As the larva matures, the body enlarges axially at the expense of the lobes, which shrink and gradually disappear.
The pilidium is helmet-shaped, with lateral lobes like ear-flaps on each side. The band runs along the margins of the helmet, and the larva itself is delicate and transparent. The juvenile develops internally in a remarkable process involving the ingrowth of a number of imaginal discs, which apply themselves to the stomach and expand and fuse to enclose it. The juvenile worm then emerges through the larval epithelium and, over a period of minutes, consumes the latter as its first meal.
Among the more advanced marine protostomes, trochophore larvae are characteristic of those traditionally classed as spiralians. This includes annelids, sipunculids, echiurids and some mollusks (e.g. chitons), though most other mollusks pass through a provisional trochophore stage. The trochophore, or something very much like it, is thus very probably a basal larva for spiralians.
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Trochophores are named for the wheel-like appearance of the main ciliary band (the prototroch), which girdles the body around its middle. The mouth opens just below the prototroch, and the anus is posterior and terminal. Beside the latter are a pair of nephridia and the mesoblast cells that later give rise to the trunk rudiment. The apex is occupied by an apical tuft, variously modified in different groups; the juvenile cerebral ganglia develop on either side of the tuft. Metamorphosis can be rapid, but is not usually dramatic. It typically involves the loss of the trochal band and adjacent tissue, which reshapes the head, and the expansion of the segmented trunk. Some trochophores (e.g. serpulids, echiurids and Polygordius, and archiannelid) have an accessory feeding band, the metatroch, just below and parallel to the prototroch. The prototroch and metatroch cilia beat in reverse direction, and food particles are propelled along the groove between them (the food groove) to the mouth. This arrangement is referred to as an opposed-band feeding system. Other trochophores have evolved alternative feeding mechanisms, often involving fleshy lips or specialized oral ciliature. It is not clear whether the metatroch is a primitive feature of basal spiralians or a later specialization, but this is a potentially interesting topic for future research.
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Among mollusks, non-feeding larvae, some trochophore-like, others less so, are found among the aplacophorans and chitons. Gastropods and bivalves have veliger larvae, which differ from each other somewhat because of the different shell form. The figure shows the gastropod veliger. It has the basic topology of a trochophore, but with an expanded ventral foot region with an operculum, a shell secreted to enclose the visceral mass, and an expanded pair of velar lobes with a food groove running along its edge. The expanded velar lobes greatly lengthen the surface available for food collection, but the arrangement is otherwise essentially the same as in trochophores with a metatroch, and the molluscan velum is presumably derived from this.
The sequence below shows, in schematic form, the relation between the various spiralian larvae, beginning with a hypothetical planula-type ancestor with a posterior mouth and cilia. Whether or not these were ever organized as a separate band in the ancestral larva, as shown, the direction of beat is always towards the mouth. Moving the mouth forward along the side of the body, as in Müller's larva or the pilidium, would then define an effective ventral surface. Adding an anus and metatroch then gives us, in sequence, the trochophore and veliger.
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