FIRST DORSAL FIN. Remove the mass of muscles on both sides of the base of the fin down to the vertebral column. The principal cartilages of the fin lie in the median connective tissue septum which separates the dorsal musculature of the two sides of the body. The basal cartilages of the fin are attached to the vertebral column by means of this septum. It is best to remove the underlying portion of the column with the fin. The cartilages can then be scraped perfectly clean. The skeleton of the fin is composed of three rows of cartilages: (1) a basal row consisting of one very large, flat plate and two or three smaller ones posterior to it; (2) an intermediate row of several plates of nearly equal size; and (3) a distal row of several very small plates. The intermediate and distal rows extend beyond the body musculature into the base of the fin. The remainder of the fin is supported by the dermal rays. In front of the cartilages which have been mentioned is the strong spine of dentine (see p. 5), with its free portion sheathed by an enamel-like covering. SECOND DORSAL FIN. Remove this from the body in the same manner as the first dorsal. Its structure follows the same general plan, the differences being minor ones of shape, size, and number of plates. Several thin cartilaginous plates are sometimes formed in the median septum in front of the spine. CAUDAL FIN. Only one side of the caudal fin should be cleaned, as when both sides are cleaned there is danger of breaking the delicate cartilages. The cartilaginous skeleton of the caudal fin consists of a row of slender rods along the dorsal side of the vertebral column, extending to its tip. There are no cartilaginous elements in the fin ventral to the vertebral column. By far the greater part of the caudal fin is supported by the two layers of horny fin-rays only. MUSCULATURE Dissect the skin off the head, neck, and body to back of the pectoral fins. Observe first the musculature of the dorsal side of the neck and of the body back of the bases of the pectoral fins, noticing that it is composed of narrow, zigzag bands, called _myomeres_. Where these are fully developed they extend from the mid-dorsal to the mid-ventral line. Note carefully the relation of corresponding myomeres of the two sides, the exact course of a single myomere, and the direction of the muscle fibres in a typical myomere. Observe also that the muscles above the level of the vertebral column form a thick mass, which is frequently referred to as the dorsal musculature; the muscle below this level may be correspondingly referred to as the ventral musculature. As the muscles described below are dissected the mechanical effect of each should be determined.