8 GENERAL SURVEY OF THE ANIMAL KINGDOM. 
It does not matter much whether we retain the familiar title Verte 
brata, or adopt that of Chordata, provided that we recognise—(1) that 
it is among Fishes first that separate vertebral bodies appear in the 
supporting dorsal axis of the body ; (2) that, as a characterdstzc, the 
backbone is less important than the notochord, which precedes it in 
the history alike of the race and of the individual. Nor need we 
.object to the popular title backboned, if we recognise that the adjective 
“bony ” is first applicable among Fishes, and not to all of these. 
The essential characters of Vertebrates may be summed up in the 
following table, where they are contrasted, somewhat negatively, with 
what is true of Invertebrates :— 
‘BACKBONELESS,” INVERTEBRATE 
or Non-CHORDATE. 
‘“BaCKBONED,” VERTEBRATE 
oR CHORDATE. 
If there is a nerve-cord, it is ventral, 
No internal dorsal axis. 
No gill-slits. 
The eye is usually derived directly from 
the skin. 
The heart, if present, is dorsal. 
The central nervous system—brain and 
spinal cord—is dorsal and tubular. 
There is a dorsal supporting axis or 
notochord, which is in most cases 
replaced by a backbone. 
Gill-slits or visceral clefts open from the 
sides of the pharynx to the exterior. 
In fishes, and at least young amphi- 
bians, they are associated with gills, 
and are useful in respiration; in 
higher forms they are transitory and 
functionless, except when modified 
into other structures. 
The essential parts of the eye are formed 
by an outgrowth from the brain. 
The heart is ventral. 
INVERTEBRATES, OR BACKBONELESS ANIMALS 
Molluscs.—If we take the concentration of the nervous 
system as a useful criterion, the highest backboneless 
animals are the Molluscs. This series of forms includes 
Bivalves, such as cockle and mussel, oyster and clam; 
Gasteropods, such as snail and slug, periwinkle and whelk ; 
Cephalopods, such as octopus and pearly nautilus. 
Unlike Vertebrates, and such Invertebrates as Insects 
and Crustaceans, Molluscs are without segments and 
without appendages. A muscular protrusion of the ventral 
surface, known as the “foot,” serves in the majority as ap 
organ of locomotion. In most cases a single or double 
fold of skin, called the “ mantle,” makes a protective shell. 
The nervous system has three chief pairs of nerve centres 
or ganglia. In many cases there are very characteristic 
free-swimming larval stages. 
