STRUCTURE OF THE VERTEBRATES 39 



The sharks are the most primitive fish known, some of them 

 having six or seven gills. The most anterior slit has become 

 modified in the elasmobranchs and a few higher fish into a 

 small opening, the spiracle. This structure is of importance in 

 the evolution of the land vertebrates. With the exception of 

 these two families, the fish have five gills. 



The dogfish, a small shark, is an ''epitome of vertebrate 

 anatomy", and is usually used in courses of comparative anat- 

 omy for dissection. 



1. External Anatomy. The dogfish has a fusiform body adapt- 

 ing it for rapid locomotion in the water. A pointed rostrum 

 projects in front of the mouth; and at the end of the tapering 

 body the vertebral column turns up to form an asymmetrical, 

 heterocercal, tail. The pelvic fins are in the primitive position 

 immediately anterior to the anus; and, with the pectoral fins, 

 are used as balancing organs in swimming. The body is covered 

 with dermal denticles which have a wide basal plate with a 

 spine which projects through the skin to the outside. The cloaca 

 is the outlet of the digestive and urino-genital ducts. 



2. Internal Anatomy. The chondrocranium is roofed and en- 

 closes the brain. The jaws are not fused to the chondrocranium 

 but are suspended by ligaments and cartilages. The pharynx 

 is large. As the food is swallowed whole the stomach carries on 

 a very large part of the protein digestion, and the absorptive 

 small intestine is highly coiled and fused to form a spiral valve. 

 The oviducts, into which the eggs pass from the ovary, are large 

 and vascular. As the eggs remain in the oviducts until the 

 young have absorbed the yolk mass, the vascular supply un- 

 doubtedly assists in respiration and the removal of waste 

 products. 



Teleostomi. The name refers to the fact that the mouth of 

 these fish, like that of the higher vertebrates, has the jaws en- 

 cased in bone, with the upper jaw fused to the skull. This 

 makes the upper jaw immovable and is a definite advance 

 over the elasmobranch condition. In this group the dermal 

 denticles have developed into dermal plates or scales, and in 

 the head region the plates cover the chondrocranium as the 

 dermal bones of the skull. The sub-class is very large and 



