152 ELEMENTS OF BIOLOGY 



closely by a tough membrane, which is anchored to the bone by 

 fibres that are deeply rooted in the bone substance. This covering 

 is the teriosteum; the muscles are attached to this membrane, so 

 that it serves as the means of attaching the muscles to the bone. In 

 case of injury to the bone, certain cells in the periosteum become 

 active and invade the break and surround and fill it with a splice of 

 bony tissue. It is to be understood that while bone to outward 

 appearance is a rigid and solid substance, it is alive and contains 

 living bone cells and the circulation of blood and lymph necessary 

 to their maintenance. The appearance of a section of bone under the 

 microscope is shown in an accompanying figure (Fig. 97). 



The Human Architectural Plan. The indebtedness of the 

 human body for its major architectural features may be briefly sum- 

 marized. Bilateral symmetry appears very low in the scale of struc- 

 tural complexity; the most simple Metazoa that are built on this 

 plan are the flatworms, the phylum Platyhelminthes. The three- 

 layer, or triploblastic pattern also appears first conspicuously in this 

 group. The body plan of a tube-within-a-tube with a cavity, the 

 coelom (see p. 137), between the two tubes and with the inner or 

 digestive tube opening at both ends first appears in the round 

 worms, the phylum Nemathelminthes. The partitioning of the 

 coelom into a pericardial sac and a peritoneal cavity occurs in fishes. 

 The further division of the peritoneal cavity into the pleural and 

 peritoneal cavities occurs in the lower mammals. The division of the 

 body into segments of metameres is most highly developed in the 

 Annelida and continues to be a characteristic in the Arthropoda and 

 chordates. Various and diverse types of exoskeletal structures appear 

 in the more simple animals, but true bone appears first in the lower 

 vertebrates. The human endoskeleton appears to owe its origin to 

 an association between exoskeletal elements and cartilage elements 

 that first appear low in the phylum Chordata. 



This closes a rapid review of the major characteristics of the 

 animal as a whole. The next task is the examination of the various 



