Fig. 148. Polygordius larva.
(From Alex. Agassiz.)
The general mode of development of Chætopod larvæ is similar to the above except in details, which are however no doubt often of great importance. The history of the larvæ may be conveniently treated under three heads. (1) The form of the primitive unsegmented larva; (2) the arrangement of the cilia on the unsegmented larva, and on the larva at later stages; (3) the character of the metamorphosis and the development of the permanent external organs.
A larva similar to the Polygordius larva with a greatly developed præ-oral lobe is widely distributed amongst the Annelids.
Fig. 149. Larva of Phyllodoce. (From Alex. Agassiz.)
An almost identical form is that of Nepthys scolopendroides (Claparède and Metschnikoff, No. [336]); that of Phyllodoce ([fig. 149]) is also very similar, and that of Saccocirrus (Metsch. and Clap. No. 336, Pl. XIII. fig. 1), a very primitive form most nearly related to Polygordius, clearly belongs to the same type. Many other larval forms, such as that of Spio fuliginosus (Metsch. and Clap. No. [336]), Terebella, Nerine, etc., also closely approach this form.
Other really similar forms at first sight appear very different, but this is mainly owing to the fact that their præ-oral lobe never attains a considerable development. Its smallness, though obviously of no deep morphological significance, at once produces a very different appearance in a larva.
A good example of a larval form with a small præ-oral lobe is afforded by Capitella, which is figured by Clap. and Metsch. (No. [336], Pl. XVII. fig. 2). The imperfect development of the præ-oral lobe is also generally characteristic of the Oligochæta. The persistence of a relatively large præ-oral lobe for so long a time as in Polygordius is very unusual.
The arrangement of the cilia in Chætopod larvæ has been employed as a means of classifying them. Although a classification so framed has no morphological value, yet the terms themselves which have been invented are convenient. The terms most usually employed are Atrochæ, Monotrochæ, Telotrochæ, Polytrochæ, Mesotrochæ. The polytrochæ may again be subdivided into Polytrochæ proper, Nototrochæ, Gasterotrochæ, and Amphitrochæ.