114 VERTEBRATE ZOOLOGY 



ulation and for introducing sperm into the latter's oviducts. The 

 oggs are internally fertilized and embryonic development takes place 

 in the uterus of the mother. This is a highly speciahzed character, 

 as will be clear when the more primitive sharks are described. The 

 mouth is ventrally situated and is armed with a number of rows of 

 teeth on both upper and lower jaws. The teeth are obviously modi- 

 fied placoid scales. The body is covered all over with small scales 

 {placoid scales), with a bony plate-like base of dermal origin and a 

 sharp protruding spine covered with hard enamel of ectodermal ori- 

 gin. Between the mouth and the pectoral fins are the gill-slits 

 (phar5Tigeal clefts) each of which opens separately to the exterior. 

 The anterior gill-slit on each side is small and modified into a spiracle, 

 situated just back of the eye. Several external features discussed in 

 other connections are nasal apertures, cloaca, lateral-line organs, and 

 lastly, the tail, which is provided with a typical heterocercal caudal fin. 



The Skeletal System. — The entire skeleton is cartilaginous with 

 only a slight impregnation of calcareous matter. The cranium is a 

 chondrocranium, a solid, one-piece capsule completely inclosing the 

 brain and the principal sense organs. The cranium proper is fused 

 with paired nasal capsules, and paired auditory capsules. The 

 vertebral column consists of a series of hour-glass-shaped vertebrae, 

 with lens-shaped pieces of the original notochord between adjacent 

 vertebrae, and connected with each other by a strand of notochordal 

 tissue perforating the entire set of vertebrae like the string through 

 a chain of beads. Closely associated with the skull but not fused with 

 it, is the mandibular skeleton, consisting of an upper jaw {palato- 

 quadrate cartilage) and a lower jaw {Meckel's cartilage). Back of the 

 jaws are the visceral arches, that are composed of upper and lower 

 parts like the jaws; the first pair being specialized as the hyoid arch, 

 the five others being the more generalized bi-anchial arches that afford 

 support for the gills. The fins all have cartilaginous ray-like supports, 

 and the pectoral and pelvic limb skeletons are supported upon sim- 

 ple horseshoe-shaped girdles {pectoral and pelvic girdles) each com- 

 posed of but one piece of cartilage. 



The Alimentary System.— The mouth opens directly into the 

 capacious pharnyx, which is perforated by five gill-clefts and the paired 

 spiracles. A short oesophagus of large caliber leads into a U-shaped 

 stomach, which in turn communicates through a valvular opening, con- 

 trolled by a sphincter muscle, with the intestine (Fig. 62). The latter 



