3i8 



ZOOLOGY FOR MEDICAL STUDENTS 



CHAP. 



its inner edge (Fig. 124, B, v.a) and bearing externally the slender tapering 

 gill-rays (g) that support the thin part of the gill-septum. The originally 

 continuous hoop-like cartilage becomes segmented into a number of 

 pieces as may be seen in Fig. 133 (3, 4, ^, 6, 7). While in the case of the 

 branchial arches the details of this differentiation are of minor importance^ 

 in the case of the mandibular and hyoid arches on the other hand some 

 of them have an important bearing upon the morphology of vertebrates 

 in general and therefore must be alluded to. 



The first visceral arch or mandibular arch lies between the mouth 



Vso. VHso. 



Fig. 133. 



Cartilaginous skeleton of head and visceral arches of ScylUum, as seen from the left side. {After 

 W. K. Parker.) a, Artery ; aud, auditory capsule ; c.h, ceratohyal ; Cr, chondrocranium ; 

 h.m, hyomandibular ; I, labial cartilages ; M, Meckel's cartilage ; olf, olfactory capsule ; oph, ophthal- 

 mic ; ppf palatopterygoid cartilage ; ps.c, pre-spiracular cartilage ; Vs,Of VIls.o, superficial oph- 

 thalmic nerves ; v, vein. 



Roman numerals indicate foramina for cranial nerves ; Arabic numerals, skeleton of visceral 

 arches. 



and the spiracle. The upper portion of the cartilage, next the cranium, 

 is reduced to a functionless vestige — the prespiracular cartilage — lying 

 just in front of the spiracle. The ventral portion on the other hand 

 becomes much enlarged, supports the lower edge of the mouth with its 

 teeth, and forms the lower jaw or Meckel's cartilage (Fig. 133, M). This 

 Meckel's cartilage is of great interest as it is the primitive lower jaw of 

 the vertebrate and is always present during early stages of development 

 even in vertebrates in which the lower jaw of the adult is composed of 

 bone and different in its morphological nature. The primitive upper jaw 

 of the vertebrate — ^which is also seen typically in the Dogfish (Fig. 133, pp) 



