The sense organs are comparatively simple. A pair of very small eyes lie in the skin of the head; they are of complex structure, and to some extent remind one of the simple eyes of certain Crustacea. Behind the eyes and also on the upper surface of the animal is an unpaired organ which is usually described as olfactory in function (Figs. 103, 105). This is a ring-shaped modification of the epidermis drawn out into different shapes in the various species. The modified epidermal cells bear long cilia. The remaining sensory organs found in the group consist of clumps of modified cells scattered in round groups over the surface of the body and of the fins. The central cells of each group bear long tactile hairs, and are surrounded by supporting cells. [Illustration: FIG. 102.—Head of _Sagitta bipunctata_. A, Dorsal view; B, ventral view. × about 33. (From Hertwig.) A, _a_, spines; _b_, nerves to lateral cephalic ganglia; _c_, hooks; _d_, cephalic ganglion; _e_, olfactory nerve; _f_, optic nerve; _g_, hood; _h_, commissure to ventral ganglion; _j_, olfactory organ: B, _a_, _c_, and _g_ as in A; _k_, mouth.] The Chaetognatha are hermaphrodite, and carry the female organs in the trunk, the male in the tail. In a mature specimen the two ovaries occupy almost all the space in the trunk between the alimentary canal and the skin, and each is supported by a narrow lateral mesentery. The ovary is traversed by a oviduct which often contains spermatozoa; it is not clear how the eggs make their way into the oviduct, which seems to have {189}no internal opening and to act largely as a receptaculum seminis. The oviducts open externally on the upper side at the base of the lateral fin, close to the junction of the tail and the trunk. The cavity of the tail is divided into two lateral chambers by the extension backward of the median vertical mesentery. In each of these a testis and a vas deferens are found. The testes are solid ridges formed by the growth of the lining cells of this part of the body-cavity; the cells mature into spermatozoa, which break off and float freely in the coelomic fluid. At the breeding season the whole tail may be crowded with masses of spermatozoa, which are kept in a more or less regular circulation by the ciliated cells lining the body-wall. The vas deferens opens internally into the space where the spermatozoa lie, and at the other end into a vesicula seminis, which opens to the exterior. The position of the latter structure varies, and is of some systematic value.