Fig. 143.—A gill filament, A, of Branchiomma, B, of Dasychone. a, Axis; f, secondary filaments; o, compound eye; x, lappets.
The Capitelliformia and Opheliidae possess a pair of peculiar "ciliated pits" or "nuchal organs" at the upper side of the head, between the prostomium and peristomium, and capable of eversion (Fig. 144). They are most characteristically developed in the Capitelliformia, where each organ abuts upon a special lobe of the brain. The function of these "ciliated organs," which bear a great resemblance to those of the Nemertines, is a matter of speculation. Similar organs, in the form of simple pits or grooves, occur in many of the Nereidiformia, Terebelliformia, and others.[[326]]
Otocysts are rare. Arenicola possesses a pair at the base of the prostomium, each of which in some species retains an opening to the exterior.[[327]] They probably serve as "organs of direction" rather than of "hearing." Aricia and Polyophthalmus likewise have such organs on the prostomium; whilst Fabricia, Myxicola, Terebella, and a few others possess them in the peristomium, or in some other segment of the body.
Fig. 144.—Ammotrypane aulogaster Rathke, enlarged. (From Cuningham.) Anterior end. a, Prostomium; b, everted buccal region; c, notopodial cirrus; X, ciliated organ everted; I, II, III, first three segments.
Reproductive Phenomena.—With a few exceptions mentioned below, the Polychaeta are unisexual. The sexual cells are developed in all cases from the lining epithelium of the body-cavity. The exact spot at which this occurs varies in different cases; it may be, though rarely, on the floor of the body-cavity; it is more usually on the wall of some blood-vessel, either the ventral vessel or on branches of it; or on the many blind blood-vessels of Aphrodite. The number of such genital organs is very great in most worms, but in those presenting two regions of the body they are confined to the posterior segments (Sabelliformia, Terebelliformia, Capitelliformia). The number is very limited in Arenicola and other worms presenting but few nephridia: in the former genus there being six pairs, in Trophonia only one pair.
The following genera are hermaphrodite:—Amphiglena, Salmacina, Protula, Spirorbis, belonging to the Sabelliformia, to which must be added some Hesionidae. In this family ova and spermatozoa are developed around the same blood-vessel. But in the former group of worms (as also in Ophryotrocha) the two kinds of cells are produced in different regions of the body. Thus in Protula the anterior abdominal segments are male, the posterior ones female, while in Spirorbis the reverse arrangement holds; and in Syllis corruscans the anterior segments of the body contain eggs, whilst the posterior region contains spermatozoa, and this region separates and becomes a male worm.
The eggs and spermatozoa in the Polychaeta are discharged into the sea either by rupture of the body-wall or through the nephridia; the male and female elements unite, and the resulting fertilised eggs undergo development, either floating separately in the water, or embedded in jelly, or attached to the body or to the tube of the worm.
The result of the segmentation of the egg is a free-swimming larva known as a "Trochosphere," similar to that of Polygordius. The larvae of different species present various more or less marked departures from this type, for instead of the two girdles of cilia there may be only the anterior girdle, or there may be several complete or incomplete girdles between the two typical ones, or there may be (Chaetopterids) only a single girdle of cilia about the middle of the body, the two typical girdles being absent.[[328]] The postoral region, after elongation, generally becomes marked out into three segments, and these segments develop chaetae, which are usually temporary and specially long.