MARINE VERTEBRATES 307 



quite sufficient to enable us to decide whether any particular animal 

 is a vertebrate or an invertebrate, for it will be remembered that 

 the body of the latter has only one cavity, containing the nervous 

 system as well as the viscera, and that the nervous system is 

 commonly placed along the ventral side, but never along the dorsal. 

 In addition to this the vertebrates never have more than two pairs 

 of limbs, and these are always directed from the nervous system ; 

 and the jaws, which are appendages that move in the horizontal 

 plane in invertebrates, are, in the higher animals, portions of the 

 framework of the head and move vertically. In vertebrates, too, 

 there is always a complete blood system, consisting of a heart with 

 two, three, or four cavities, a system of arteries to convey the 

 blood to the different parts of the body, veins to return the blood 

 to the heart, and networks of fine capillaries connecting the former 

 with the latter. 



All vertebrates, at an early stage of their existence, have a 

 cartilaginous rod running through the dorsal portion of the body, 

 called the notocord. In some of the lowest animals of the division 

 this rod persists without any important alterations in structure, 

 while in the higher forms it gives place to the series of cartilaginous 

 or bony elements above referred to as the vertebrae; and the 

 arrangement of the vertebrates into their relative positions in the 

 scale of life is based largely on the degree of development of the 

 vertebral column from the notocord. Another interesting feature 

 in the development of a vertebrate is the formation of five or more 

 transverse, archlike thickenings on each side of the digestive tube, 

 just behind the head ; and, in the spaces between them, of a series 

 of slits forming a communication between the pharynx and the 

 exterior. These arches and clefts have but a brief existence in 

 many vertebrates, while in others they persist throughout life; 

 and, like other points referred to, they assist us in recognising the 

 relations of the vertebrates to one another. 



The vertebrates are divided into the following classes : 



1. Cyclostomata Lampreys. 



2. Pisces Fishes. 



3. Amphibia Frogs, Toads, Newts, &c. 



4. Reptilia Snakes, Lizards, Tortoises, &c. 



5. Aves Birds. 



6. Mammalia Mammals. 



The first of these includes only a few species, one of which is 



