elegans are free-swimming planktotrophic animals with a ciliated prototroch and metatroch
and a collar (Fig.
In all observations of living and fixed larvae, the prototroch and metatroch
appeared to consist of simple, not compound, cilia.
Sparse cilia were also located below and within the mouth of the trochophore, although they did not form the distinct band of cilia characteristic of a true metatroch
The band of very long prototrochal cilia traveled along the dorsal lip of the mouth, whereas the metatroch
traveled along the ventral lip.
Wilson, 1929, 1968; Dales, 1952; Cazaux, 1964; Smith and Chia, 1985) are ambiguous as to whether two of the ciliary bands necessary for this feeding mecha-nism--the metatroch
and food groove--are even present in sabellariid larvae.
These findings suggest that the secondary ciliated band in the veliger is not homologous to the metatroch
found in the trochophore larva of other spiralians, such as the annelid Polygordius lacteits (reportedly derived from 3c and 3d; Wollereck.
High-speed video demonstrated that prototroch and metatroch
cilia of a gastropod veliger, Lacuna vincta, captured particles by overtaking and intercepting them and that the velar cilia caught and transported particles larger than the food groove, though these were not ingested.
Most annelid larval forms, feeding or not, can be readily related to the trochophore ground plan, which Nielsen (2001, 2004) defined as a larva bearing an apical tuft, a preoral ciliated band derived from trochoblasts (prototroch), an adoral ciliary zone, and postoral metatroch
, gastrotroch, and telotroch, all of which bands consist of multiciliate cells with compound cilia.
Surprisingly, Pernet (2003) observed complete downstream-collecting ciliary systems with pro-totroch, adoral ciliary zone, and metatroch
in larvae of the sabellid polychaetes Schizobranchia, Demonax, Myxicola, and Pseudopotamilla.
According to Nielsen (1995), the ancestral, feeding trochophore is defined by six characters: prototroch (pre-oral ciliary band), metatroch
(post-oral ciliary band), ciliated food groove between the prototroch and metatroch
, apical sensory organ, functional gut, and pair of proto-nephridia.
The inference that the common ancestor of serpulids and sabellids had a feeding larval stage relies primarily on the observation that feeding larvae of sabellariids and serpulids capture particles using similar systems of three parallel ciliary bands, the prototroch, food groove, and metatroch
However, tracing the fate of both these trochal bands ahead in ontogeny reveals that they come to lie well within the segmented trunk of the larva, contrary to the definition of a metatroch
as a presegmental structure lying on the peristomium (Rouse, 1999).