THE SKELETON. 27 



Shells mostly composed of carbonate of lime. Lingula an 

 exception (phosphate of lime). 



Shells grow in thickness and in length; the former is se- 

 cured by secretion from the mantle everywhere excepting at 

 the edge. Here growth in length takes place, as the shell 

 extends. The shell may be variously colored by the secre- 

 tion of pigment cells. Growth is periodic. Lines of growth, 

 therefore, furnish an index to age. The muscles employed 

 in closing valves of certain molluscs move their bases at each 

 time of growth, so that the same relative position of the parts 

 of the shell may be maintained. Byssus is the hardened, 

 cord-like secretion of a peculiar gland seen in certain bivalve 

 molluscs. Fastened by one extremity to some submerged 

 object, the other being in relation to the shell, the individual 

 obtains through the byssus a secure anchorage. 



ARTICULATA. Skeleton external, composed of a number 

 of rings joined by fibrous structures. May be chitiuous or 

 calcareous ; diffuse or compact. 



Annulations obscure in many Entozoa and Annelida. 



Anus terminal in Crustacea, Myriapoda, Arachuida, In- 

 Becta; dorsal and generally terminal in Annelida; not con- 

 stantly terminal in Rotifera or Entozoa. Rudimentary forms 

 of skeleton in Entozoa. Extraneous skeletons in Tubicola 

 and in larva of Phrygana. 



Structure epidermoid, as in Lytta ; tubular in Scolopendra 

 (centipede), Limulus (king-crab), and Balanus (acorn barnacle). 



Certain articulates, as Crustacea and some larval forms of 

 Insecta (Cicada], shed their skeletons periodically. This is 

 called exigesis. 



Limbs and wings are appendages to the segments. Limbs 

 may be without joints (setae), as in Annelida, or jointed (ar- 

 thropod), as in Insecta. "Wings are confined to Insecta. 



VERTEBEATA. The vertebrate skeleton is a central seg- 

 mented, longitudinal axis. Of two kinds, cartilaginous and 

 osseous. In the cartilaginous form, confined to certain fishes, 

 the chorda dorsalis is persistent. In the osseous form assem- 

 blages of bones are seen, developed for the most part from a 

 cartilaginous basis. Those arising from another source are 



