170 



AFFINITIES AND GEOGEAPHICAL 



FIG. 85. 



Ecliiurus vulgaris. 



genus Fistularidce, animals externally worm-like, and pos- 

 sessing the rudiment of a heart, with red blood in the arteries. 



The reader cannot fail to have been 

 struck by the greater number of 

 forms passed through in this line, 

 in comparison with any other, be- 

 fore leaving the radiate sub-king- 

 dom ; but, in reality, the echinoder- 

 rnata, though of radiated form, are 

 much superior to the rest of that 

 division in their organization, which 

 is, if not complicated in the usual 

 sense of naturalists, full of extremely 

 curious minute work. Their whole 

 destiny seems to be of a high kind ; 

 for in the stone record their line of 

 forms stands parallel with others, in 

 which the whole of the three lowest 

 sub-kingdoms are passed through. 

 Polypiarian animals and Encrinites 

 appear in the Silurian and many sub- 

 sequent formations ; at the commencement of the carbonigenous 

 era, the latter are so abundant that we walk over large tracts 

 of country, where the rocks beneath our feet are almost wholly 

 composed of their remains. The Asteriadse appear in the 

 upper Silurians, and are but faintly seen until the Lias, when 

 they become conspicuous. In the Oolite, the Echinidse make 

 their appearance. These are the last which we could expect 

 to be preserved in rocks, as the higher families possess no hard 

 parts ; otherwise we might perhaps have seen the succession of 

 this class of fossils continued into the Holothurise and Fistu- 

 laridae. It cannot fail to be noticed how well the progression 

 of forms agrees with the order of their appearance in the geo- 

 logical ages. 



The ground is now cleared for the two grand series of In- 

 vertebrate animals, and first of the AETICULATA. These are 

 generally describable as animals " composed of a succession of 

 rings, formed by the skin or outward integument, which from 

 its hardness constitutes a kind of external skeleton ;" one class, 

 however, the Annelides, have no hard investment. The pedi- 

 gree of the Articulata is very brief. The embryo in most 

 classes passes at once from the monad to the worm form, and 

 then the articulate character is assumed. It can therefore 



