172 THE HISTORY OF CREATION. 



marine lilies and stars which are given in the following 

 table, illustrate the laws of progress and differentiation in a 

 striking manner. In each succeeding period of the earth's 

 history we see the individual classes continually increasing 

 in variety and perfection. (Gen. Morph. ii. Plate IV.) 



The history of three of these classes of Star-fish is very 

 minutely recorded by numerous and excellently preserved 

 fossils, but on the other hand, we know almost nothing of 

 the historical development of the fourth class, that of the 

 Sea-cucumbers (Holothuriae). These curious sausage-shaped 

 Star-fish manifest externally a deceptive similarity to 

 worms. (Fig, D, Plates VIII. and IX.) The skeletal struc- 

 tures in their skin are very imperfect, and hence no distinct 

 remains of their elongated, cylindrical, worm-like body could 

 be preserved in a fossil state. However, from the compara- 

 tive anatomy of the Holothurise, we can infer that they 

 have arisen, by the softening of the cutaneous skeleton, 

 from members of the class of Sea-virchins. 



From the Star-fish we turn to the fifth and most highly 

 developed tribe of the invertebrate animals, namely, the 

 Ijhylum of Artieulata, or those with jointed feet (Arthro- 

 poda). As has already been remarked, this tribe corresponds 

 to Linnteus' class of Insects. It contains four classes : 

 (1) the genuine six -legged Insects, or Flies ; (2) the eight- 

 legged Spiders ; (3) the Centipedes, with numerous pairs 

 of legs ; and (4-) the Crabs, or Crustacea, whose legs vary in 

 number. The last class breathe water through gills, and may 

 therefore be contrasted as the main-class of gill-breathing 

 Arthropoda, or Gilled Insects (Carides), with the three first 

 classes. The latter breathe air by means of peculiar wind- 

 pipes, or tracheae, and may therefore appropriately be united 



