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, antennae 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, only 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 oesophagus, 

 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 



