29 



basals are three rows of rod-like radial cartilages, the proximal row 

 being articulated to the basals. 



PELVIC GIRDLE AND FIN. Remove the pelvic girdle from the body 

 with the pelvic fins attached, and clean away the muscles. 



The pelvic girdle consists of an almost straight bar of cartilage, 

 slightly thicker at its middle than at its ends, which lies trans- 

 versely in the ventral wall of the abdomen. To each end is at- 

 tached a long basal cartilage which lies in the fin, close to and 

 parallel with its inner margin. A proximal series of slender radial 

 cartilages is attached to the lateral side of the basal; a distal series 

 of very short radials lies outside of the first series, while the portion 

 of the fin beyond these is supported by the dermal fin rays. 



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 por- 

 tion 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 inter- 

 mediate 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 deli- 

 cate cartilages. The cartilaginous skeleton of the caudal fin con- 

 sists 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 correspond- 

 ing myomeres of the two sides, the exact course of a single myomere, 

 and the direction of the muscle fibres in a typical myomere. Ob- 

 serve 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 de- 

 termined. 



