312 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



the two valves are hinged on one another (Testi- 

 cardiiies or Articulata). The dorsal valve may 

 give off hard processes, which project into the cavity 

 between the two valves, and which has a spiral or 

 looped form (Fig. 128) ; they serve as a means of 

 support for the highly-developed " arms " of the 



Fig 128. Dorsal Valves of various Brachiopods, seen from within, to 

 'show the loop (1). A, Terebratula ; B, Terebratulina ; c, Terebratella ; 

 D, Bouchardia; E, Megeiiia ; F, Argiope; h, Hinge ; I, loop; s, 

 septum. (After Davidson.) 



Brachiopoda. In some (e.g. Terebratula) the sub- 

 stance of the shell is traversed by tubular prolonga- 

 tions of the contained mantle. Scattered calcareous 

 spicules are to be found in the integument. 



Among the Chordata, a well-developed skeleton 

 appears only in the Verteforata, where it attains to 

 considerable complexity; the more characteristic is 

 internal, but an external skeleton is sometimes also 

 present. 



All Vertebrata at some period of their lives, the 



