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 vertebræ; 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 found in our seas, and will receive a short notice here. The fishes will, of course, demand a fair share of our attention. Amphibians and reptiles have no British marine representatives, and are therefore quite excluded from this work. As to the birds, although there are so many that live entirely on the sea and in its immediate neighbourhood, these have been so ably dealt with by Mr. Hudson in one of the books of this series that it would be superfluous to mention them. The mammals include a considerable number of marine species, but as only one of these--the Porpoise--is really commonly observed round our coasts, it alone will be selected for description. Lampreys and their few allies were formerly classified with fishes, but are now made to form a small class by themselves; and there is abundant reason for the separation. It will be remembered that vertebrates, in the early stages of their development, are characterised by a cartilaginous rod running through the dorsal region of the body, below the central cord of the nervous system, and that they possess a series of slits opening into the sides of the pharynx. Now, while these characteristics are usually only transitory in the vertebrates, the Lampreys and their relatives are the only animals in which they persist throughout life, and it is for this reason that they are exalted to the dignity of a class under the title _Cyclostomata_.