HYOBRANCHIAL APPARATUS OF SPELERPES 535 



medial surface of the ceratohyal and converges to a narrow 

 insertion on the ventral surface of the proximal end of the first 

 epibranchial. 



The adductor arcuum is a ribbon-like muscle which is un- 

 doubtedly a fusion of the adductores arcuum secundi et tertii of 

 other forms. It arises from the lateral projection of the second 

 basibranchial, curves around the thoracicohyoideus in a postero- 

 lateral direction and divides into two slips, the larger of which is 

 inserted into the ventral surface of the second epibranchial and 

 the smaller into the ventral surface of the third epibranchial. 



The two constrictores arcuum (interbranchiales of Driiner) are 

 small muscles, parallel to each other on the gill arches. The first 

 arises from the ventral surface of the first epibranchial, posterior 

 to the insertion of the ceratohyoideus internus, and passes in an 

 oblique posteromedial direction over the insertion of the first slip 

 of the adductor arcuum to be inserted into the ventral surface of 

 the third epibranchial, a little posterior to the insertion of the 

 second slip of the adductor. The second constrictor lies just 

 posterior to the first. It arises from the ventral surface of the 

 second epibranchial and is inserted into the ventral surface of 

 the third. 



Five levatores arcuum are represented in the Sperlepes larva, 

 all of which arise from a fascia just beneath the dorsal integument. 

 The first, as mentioned above, is inserted into the first epi- 

 branchial along with the ceratohyoideus externus. The second 

 and third are small muscles, inserted into the dorsal surface of 

 the distal ends of the second and third epibranchials, respec- 

 tively. The fourth (the dorsal half of digastricus pharyngeus 

 of Goppert and dorsobranchialis 4 of Wilder) is a larger muscle, 

 inserted into the tendinous raphe which represents the fourth 

 epibranchial, and the fifth, the dorsotrachealis of lunged forms, 

 is inserted with its fellow in the midline dorsal to the pericardium. 



Two depressors arcuum, the third and fourth, occur. The 

 depressor arcus tertius (anterior half of hyotracheales, or pharyngo- 

 branchialis 3) arises from the medial border of the third epi- 

 branchial and is inserted with its fellow in the midline dorsal to 

 the pericardium. The fourth (posterior half of hyotrachealis. 



