i6 LABORATORY DIRECTIONS 



In this species of shark there are seven cartilaginous arches 

 nearly surrounding the anterior part of the alimentary canal 

 and supporting its walls. The first two visceral arches are in 

 close connection with the cranium. The details of structure of 

 the visceral skeleton are difficult to study on mounted skeletons, 

 but with the aid of a chart and the following description make 

 out as much of the anatomy as possible. 



1. Mandibular Arch. — Both upper and lower jaws are 

 derived from the first visceral arch. The upper jaw is a strong 

 bar of cartilage formed by the union of paired ptery go-quadrate 

 cartilages meeting in the median line. The lower jaw is made 

 up wholly of MeckeVs cartilage. 



2. Hyoid Arch. — This, the second visceral arch, lies imme- 

 diately posterior to the lower jaw. Its ventral half forms the 

 hyoid apparatus supporting the floor of the mouth. The dorsal 

 half connects the hyoid apparatus and both jaws to the cranium. 

 The hyoid consists of five cartilages: a single mid- ventral 

 basihyal; two ceratohyals which are attached to the ends of the 

 basihyal and extend parallel to the lower jaw; and two hyo- 

 mandibular cartilages attached to the corners of the cranium 

 above and supporting both jaws. The ceratohyals and hyo- 

 mandibulars bear gill rays^ slender cartilaginous rods which 

 support the first gill cover. 



3. Gill Arches. — There are five arches which support the 

 soft structures of the gills. Theoretically each gill arch should 

 contain nine separate cartilages, but this ideal arrangement is 

 never retained by all of the arches. 



Read up on the evolution of the visceral skeleton. 



Skull of a Ganoid Fish 



Examine the head of a gar-pike or other ganoid fish. The 

 ganoids show considerable advance over the elasmobranchs in 

 the development of the skull. Large dermal scales cover the 

 chondrocranium and form the roof and sides of the cranium. 

 As some of these large scales correspond to the superficial bones 

 of higher vertebrates, they have received the same names. 

 The pterygo-quadrate cartilage of the upper jaw and Meckel's 

 cartilage of the lower jaw have also become encased with 

 dermal scales, which form the superficial bones of the jaws. 



