=The Sense of Touch.=--The sense of touch is better developed among fishes. Most of them flee from contact with actively moving objects. Many fishes use sensitive structures as a means of exploring the bottom or of feeling their way to their food. The barbel or fleshy filament wherever developed is an organ of touch. In some fishes, barbels are outgrowths from the nostrils. In the catfish the principal barbel grows from the rudimentary maxillary bone. In the horned dace and gudgeon the little barbel is attached to the maxillary. In other fishes barbels grow from the skin of the chin or snout. In the goatfish and surmullet the two chin barbels are highly specialized. In _Polymixia_ the chin barbels are modified _branchiostegals_. In the codfish the single beard is little developed. In the gurnards and related forms the lower rays of the pectoral are separate and barbel-like. Detached rays of this sort are found in the thread-fins (_Polynemidæ_), the gurnards (_Triglidæ_), and in various other fishes. Barbels or fleshy flaps are often developed over the eyes and sometimes on the scales or the fins. [Illustration: FIG. 90.--Goatfish, _Pseudupeneus maculatus_ (Bloch). Woods Hole.] The structure of the lateral line and its probable relation as a sense-organ is discussed on page 23. It is probable that it is associated with sense of touch, and hearing as well, the internal ear being originally "a modified part of the lateral-line system," as shown by Parker,[8] who calls the skin the lateral line and the ear "three generations of sense-organs." The sense of pain is very feeble among fishes. A trout has been known to bite at its own eye placed on a hook, and similar insensibility has been noted in the pike and other fishes. "The Greenland shark, when feeding on the carcass of a whale, allows itself to be repeatedly stabbed in the head without abandoning its prey." (GÜNTHER.) FOOTNOTES: [8] See Parker, on the sense of hearing in fishes, American Naturalist for March, 1903. CHAPTER IX THE ORGANS OF REPRODUCTION =The Germ-cells.=--In most fishes the germ-cells are produced in large sacs, ovaries or testes, arranged symmetrically one on either side of the posterior part of the abdominal cavity. The sexes are generally but not always similar externally, and may be distinguished on dissection by the difference between the sperm-cells and the ova. The ovary with its eggs is more yellow in color and the contained cells appear granular. The testes are whitish or pinkish, their secretion milk-like, and to the naked eye not granular. [Illustration: FIG. 91.--Sword-tail Minnow, male, _Xiphophorus helleri_ Heckel. The anal fin modified as an intromittent organ. Vera Cruz.]