Fig. 152. Larva of Nerine, with provisional setæ. (From Alex. Agassiz.)
In many cases the setæ appear before there are any other visible rudiments of the feet (e.g. Lumbriconereis); while in other cases the reverse holds good. The gills are usually the last parts to appear.
Not only does the mode of development of the feet differ greatly in different types, but also the period. The appearance of setæ may afford the first external indication of segmentation, or the rudiments of the feet may not appear till a large number of segments are definitely established.
A very considerable number of Chætopod larvæ are provided with very long provisional setæ ([figs. 152] and [153]). These setæ are usually placed at the sides of the anterior part of the body, immediately behind the head, and also sometimes on the posterior parts of the body. In some instances (e.g. [fig. 153]) they form the only appendages of the trunk. Alex. Agassiz has pointed out that setæ of this kind, though not found in existing Chætopods, are characteristic of the fossil forms. Setæ of this kind are found in chætopod-like larvæ of some Brachiopods (Argiope, [fig. 136]).
Fig. 153. Embryo Chætopod with provisional setæ. (From Agassiz.)
It is tempting to suppose that the long provisional bristles springing from the oral region are the setiform appendages handed down from the unsegmented ancestors of the existing Chætopod forms. Claparède has divided Chætopod larvæ into two great groups of Metachætæ and Perennichætæ, according as they possess or are without provisional setæ.
With reference to the head and its appendages it has already been stated that the head is primarily formed of the præ-oral lobe and of the peristomial region.