THE SKELETON. 23 



protrusile foot, and in curvature of alimentary canal. Appa- 

 rently annelidous in shape of shell, and red color of blood, as 

 well as annulated form of embryo. 



The following forms have, until recently, held doubtful 

 positions among classes of Vertebrata : 



Lepidosiren. Really piscine in arrangement of gill arches, 

 in construction of scapular arch, and fin rays, and in single 

 auricle to heart. Apparently reptilian in external gill and 

 position of nares. 



Labyrinthodon. Really batrachian in presence of double 

 occipital condyle, parasphenoid and 'girdle' bones. Appa- 

 rently saurian in length of ribs, and development of dermal 

 plates. 



Archceopleryx. Really avine in union of tarsal and rneta- 

 tarsal elements, and presence of feathers. Apparently saurian 

 in prolonged production of vertebrae in a caudal series, pres- 

 ence of teeth, and construction of pelvis. 



Ornithorhynchus. Really mammalian in presence of mam- 

 mary gland, direct articulation of lower jaw to skull, complete 

 diaphragm, and the fixity of inferior strait of pelvis. Ap- 

 parently avine in presence of webbed foot, spur, and duck- 

 like bill. 



III. 



SKELETON, 



THE SKELETON is the framework of the body. It protects 

 the internal organs, while affording points of attachment to 

 ligaments and muscles. It may be diffuse or compact. 



The diffuse is composed of sclerous particles scattered 

 throughout the tissues, having no fixed union with one an- 

 other, and little or no power to retain the general form of 

 animal after the removal of the soft parts. The compact is 

 either composed of a solid mass encompassing the organism 

 in a single piece, as in Helix (snail), or, being made up of ar- 



