KINE TO GENESIS. 



271 



causes, and no deposits would take place in the val- 

 leys themselves, because they would not be subject 

 to external friction, and their continual flexion would 

 prevent any such deposits. Should such a deposit 

 take place in the valleys, there would be a stiffening 

 of the whole surface, which would defeat motion. In 

 fact, in the leecli the cuticle is already much thicker 

 on the crests of the folds than in the valleys. 



"In the more primitive Crus- 

 tacea, we find the animal made up 

 of rings extending over the whole 

 length of the body, similar to the 

 rings of the leech, save that there 

 is but one ring to one somite, and 

 instead of a perpendicular valley 

 between the folds, this valley has 

 an inward and a forward direction, 

 allowing the anterior edge of a 

 caudad ring to fit into the posterior 

 edge of a cephalad ring. 



"In the higher Crustacea, sev- 

 eral of the anterior rings have co- 

 alesced and form a solid shield 

 which is known as the carapace. 

 This has no doubt arisen by the 

 lessening of the action between the 

 anterior rings v/hen the posterior portion of the body 

 became the moie active propelling organ. As the ac- 

 tion ceased forward the valleys came to rest, and be- 

 came exposed to friction and pressure, and conse- 

 quently a deposit of calcareous matter took place pro- 

 ducing the stiffening above hinted at. 



"The formation of jointed appendages from para- 

 podic paddles of the annelids can be followed out in 



Fig. 64. — Diagram- 

 matic representation of 

 the rings of a primitive 

 crustacean, showing tlie 

 action of the muscles. 



