A larva similar to the Polygordius larva with a greatly developed præ-oral lobe is widely distributed amongst the Annelids. 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. [FIG. 149. LARVA OF PHYLLODOCE. (From Alex. Agassiz.)] 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. The atrochæ contain forms (fig. 139) in which the larva is at first coated by an uniform covering of cilia, which, though it may subsequently disappear from certain areas, does not break up into a series of definite bands. The monotrochæ or cephalotrochæ are larvæ in which only a single præ-oral ring is developed (fig. 150 B). [FIG. 150. TWO CHÆTOPOD LARVÆ. (From Gegenbaur.) _o._ mouth; _i._ intestine; _a._ anus; _v._ præ-oral ciliated band; _w._ peri-anal ciliated band.] In the telotrochæ there is present a præ-oral and a post-oral, _i.e._ peri-anal ring (fig. 150 A); the latter sometimes having the form of a peri-anal patch. The polytrochæ are segmented larvæ with perfect or imperfect rings of cilia on the segments of the body--usually one ring to each segment--between the two characteristic telotrochal rings. When these rings are complete the larvæ are polytrochæ proper, when they are only half rings they are either nototrochæ or gasterotrochæ. Sometimes there are both dorsal and ventral half rings which do not however correspond, such forms constitute the amphitrochæ.