GRADATIONS WITHIN THE TYPE. 33 
of the Crustacea, Arachnida, Myriopoda, and Insecta, is 
that their bodies are constructed of sharply-defined rings 
or segments, the legs, antennz and mandibles likewise 
sharing in this segmented character. A faithful expres- 
sion of this segmentation is afforded by the nervous 
system, which lies, ladder-like on the ventral side, that is, 
beneath the intestinal canal, nearly encircling the gullet 
with its anterior loop. The display of segmentation is 
favoured by a deposit of horny substance, which gives a 
skeleton-like stiffness to the integuments. 
The direct reverse is shown in the integuments of 
the Mollusca, our mussels, snails, and cuttle-fish. For 
although so many are supplied with protecting scales 
and shells, these are mere excretions from the actual 
skin, which remains soft, and characteristically moist 
and slimy, owing to the secretions of numerous glands 
contained in it, and has an inclination to lay itself in 
folds, and form a mantle-like investment to the body. 
The body therefore remains more or less clumsy ; it pos- 
sesses none of the grace of the Articulata, and especially 
of the insect; it is destitute of segmentation, and this 
deficiency is likewise evinced in the nervous system. 
This consists only of a ring, encircling the cesophagus, 
and a few smaller ganglia. 
We shall most readily come to an understanding as 
to the Vertebrata, the family with which man is insepa- 
rably united. The essential part is the vertebral column, 
that portion of the internal and persistently bony or 
cartilaginous skeleton, in which the main portion of the 
nervous system is contained. 
It is thus established that the systematic classification 
of the animal kingdom is based on certain prominent 
