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V)iivcrdty of California Publications in Zoology [Vol. is 



II. Interarcuales (fig. 3). The intei-arcuales, the muscles of the 

 upper segments of the gill-arches, comprise two systems, of which one 

 is dorsal, and the other more lateral. The first is made up of five, 

 the second of six similar muscles. Their function is to draw forward 

 the segment of the arch upon which they are inserted. 



1. Dorsal system. The dorsal interarcuales {ia.d.'^''') are similar 

 in origin and insertion. They have their origin posteriorly on the 

 middle half of the first to the fifth pharyngobranchial cartilages respec- 

 tively and their fibers pass posteriorly and medially to be inserted on 

 the anterior border of the succeeding phar.yugobranchial segment. The 

 muscles of this series decrease slightly in size from anterior to posterior 

 as do also the pharyngobranchial segments of the arches. 



Fig. 3. Interarcuales and subspinalis muscles, Hcptanchus maculatus, dorsal 

 view (X 1^/4). eh.^-'', first to seventh epibranchial cartilages; ia. d^''', first to 

 fifth dorsal interarcuales; ia. Z.^"", first to sixth lateral interarcuales; pb.^''^, first 

 to sixth pharyngobranchial cartilages; .s. ,sp., subspinalis muscle. 



2. Lateral system. The lateral interarcuales (ia. L^"", fig. 3) are 

 external to the series of muscles described above. These muscles 

 decrease in size posteriorly and are similar except in the case of the 

 sixth. The first may be described as typical. The greater part of its 

 fibers arise posteriorly from the external part of the first pharyngo- 

 branchial segment. The remaining fibers have their origin anteriorly 

 and dorsally from the middle part of the second pharyngobranchial 

 cartilage. These unite and continue laterally to be inserted dorsally 

 on the posterior edge of the first epibranchial cartilage. The line of 

 insertion is continuous with that of the dorsal part of the inter- 

 branchial muscle. In the seventh arch there is no pharyngobranchial 

 segment, and the sixth muscle of the lateral series originates by a 



