The Capitelliformia and Opheliidae possess a pair of peculiar "ciliated pits" or "nuchal organs" at the upper side of the head, {273}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. [Illustration: 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 {274}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.