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 (Holothuriz). 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 Holothuriz, we can infer that they 
have arisen, by the softening of the cutaneous skeleton, 
from members of the class of Sea-urchins. 
From the Star-fish we turn to the fifth and most highly 
developed tribe of the invertebrate animals, namely, the 
phylum of Articulata, or those with jointed feet (Arthro- 
toda). As has already been remarked, this tribe corresponds 
to Linnzus’ 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 tracheze, and may therefore appropriately be united 
