270 COMPARATIVE ZOOLOGY. 
Crab, we see an increasing consolidation of the exoskele- 
ton, and more perfect limbs, and a tendency to concen- 
trate the body in front, and thin out behind. Along with 
these progressive steps, we notice the muscular powers be- 
coming more energetic, and the nervous ganglia condens- 
ing into larger masses. 
The subkingdom is divided into five classes: the aquat- 
ic Worms and Crustaceans, and the air-breathing Spiders, 
Myriapods, and Insects." The various forms of articulate 
life arise from the unequal development of the body-seg- 
ments and variation in the number and form of append- 
ages. 
Crass [.—Annelida. 
The Annelids, or Worms, are distinguished from all 
other Articulates by the absence of hollow articulated 
Deis 
ly \ 
HY] ZA OS 
thn \y ROR 
VE oy L A 
> 
va \ \\ NG hy ay 
/ / \ ANN Us (4 
9 ZT SEN 
EES 
Tle 
\ 
— 
cS *\ 
Xi a \ \ = 
ne esi re) 
Fia. 241.—Marine Worm (Cirratulus grandis), with extended cirri. Atlantic. 
Y }! 
