VERTEBRATES. 



151 



Some of the Articulates are shown in figs. 170 to 179. Fig. 170 is 

 a sea-shore worm ; 171, a Crab ; 172 to 177, other Crustaceans ; 178, 



Articulates, figs. 170-179.- 

 Crustaceans : 171, Crab, 



-1. Worms : 170, Arenicola piscatorum, or Lob-worm (X %). 2. 

 species of Cancer; 172, an Isopod, species of Porcellio; 173, 



an Ampbipod, species of Orcbestia ; 174, an Isopod, species of Serolis (X%); 175, 176, 

 Sappbirina Iris; 175, female, 176, male (X 6); 177, Trilobite, Calymene Blumenbachii; 

 178, Cythere Americana, of the Cypris family (X 12) ; 179, Anatifa, of the Cirriped tribe. 



another Crustacean, having a shell like a Mollusk, but showing 

 that it is a true Articulate in having its legs and antennae jointed, 

 as well as the body within the shell ; 179, representing a Cirriped, 

 is also somewhat like a Mollusk in its shell, — though articulate in 

 structure, as the legs show, and, in fact, a Crustacean. Centipedes, 

 and all Insects, as well as Worms, are other examples of Articulates. 

 The name of the sub-kingdom is from articulus, a joint. 



IV. Vertebrates. — Having (1) a jointed internal skeleton, and 

 (2) a bone-sheathed cavity along the back for the great nervous 

 cord, distinct from the cavity for the viscera : as in Fishes, Reptiles, 

 Birds, Quadrupeds. 



The skeleton is made up of vertebrae, or the bones of the verte- 

 bral column, with their appendages ; and a vertebra is the funda- 

 mental element of the structure. The bone-sheathed cavity occu- 

 pied by the nervous cord is enclosed by processes from the upper 

 (or dorsal) side of the vertebrae, and the visceral cavity by the 

 ribs, which are processes from the lower side of the vertebrae. 

 The legs and arms are appendages to the system of vertebrae and 

 ribs. 



Recapitulation. — In Eadiates the structure is radiate or flower- 

 like. In Mollusks it is bag-like and simple. In Articulates it is 

 made of a series of rings, and is composite both in the structure 

 of the skeleton and the nervous system. In Vertebrates it is made 



