212 A VERS A ND J A CKSON. [\'ol. XVII. 



chondroidal bars (PI. XXII, Fig. 6, cs, ci). These bars are 

 somewhat flattened dorso-ventrally and serve for the attach- 

 ment of muscles and fascia. The superior bar is somewhat 

 larger and placed slightly anterior to the inferior bar. These 

 bars are not true cartilage but are composed of a chondroidal 

 tissue similar to that of the posterior segment of the basal 

 plate. They are not, therefore, to be regarded as skeletal 

 derivatives of the visceral or branchial arches, but simply as 

 chondroidal modifications {i.e., condensations of connective 

 tissue) in the muscular fascia. 



The gill cartilages (PI. XXII, Fig. 6, gb) are a remarkable 

 series of small cartilaginous bars which are so bent as to en- 

 circle the walls of the external gill passages lying near the 

 external gill openings. The broad faces of the bands, when 

 straightened out, present a somewhat semilunar shape (see 

 Fig. 12), being composed of a wide central portion and slender 

 extremities. The central portion is often perforated, as shown 

 in Fig. 12. Of the lateral pieces, the outer is longer than the 

 inner. When in position the wide central portion of the band 

 is applied to the posterior surface of the tube, the band sur- 

 rounding it so that the ends approach each other on the ante- 

 rior surface of the gill passage. The ends are at a slightly 

 lower level than the central portion of the gill cartilage, and 

 consequently nearer the mouth of the passage. In many cases 

 the ends of these gill cartilages fuse together, forming a com- 

 plete ring around the tube. There is much variation in differ- 

 ent specimens in this respect. It is evident that these gill 

 cartilages serve to keep the mouths of the gill passages open 

 under ordinary circumstances. 



Since the myotomes and gills of the opposite sides are not 

 opposite but alternate, the gill bars, of course, have the same 

 arrangement. 



The position of the gills and gill bars, however, does not 

 seem to have any fixed relation to the position of the myotomes, 

 for the openings of the external passages correspond sometimes 

 to myotomes, sometimes to intermuscular septa. They are, 

 therefore, neither regularly opposite to, nor alternate with, the 

 myotomes. 



