ABSTRACT
Larvae can be broadly defined as a structural state that occurs between the onset of the morphogenesis following embryonic development (cleavage, blastula, gastrula) and the metamorphosis to the adult body form (Hickman 1999). This covers larvae that are feeding or non-feeding, brooded by the parent in some form, or freely developing in the water. This definition also allows for larvae that go through a rapid or catastrophic metamorphosis to the adult form (e.g. Owenia) as well as those that gradually change into the adult (many directly-developing annelids). This is quite useful in the context of this chapter, since developing stages of annelids are marvellously variable in form. There have been previous comprehensive reviews that have highlighted this diversity (e.g. Schroeder and Hermans 1975; Bhaud and Cazaux 1982, 1987), and it is somewhat unfortunate that this diversity has not been generally appreciated. This may be because one particular (and rare) kind of annelid larval form, the 'classical' trochophore with opposed-band feeding (see below), tends to star in biology textbooks and in discussions on animal evolution (reviewed by Nielsen 1995). In this chapter the diversity of annelid larvae is reviewed, terminology outlined and the evolution of the various larval forms and ciliary bands is discussed.
