ORGAXIZATIOX OF ORGAN-SYSTEMS 41 



endocrine system , and include portions of the reproductive organs 

 and pancreas, the hypophysis, pineal body, suprarenal, thyreoid, 

 parathyreoid, and thymus glands. 



General Organization 



It has already been pointed out that all chordates have a 

 structural ground plan involving the possession of an axial skeleton 

 in the form of a notochord, a dorsal tubular central nervous system, 

 and a series of gill clefts leading from the alimentary canal to the 

 exterior, and that all vertebrates (which constitute a subphylum 

 of the chordates) are basically segmented. 



The rabbit being a vertebrate, its organ-systems are disposed 

 in conformity with these fundamental principles. 



(1) Axial orientation. Associated with the elongated shape 

 of the body in most animals there is a general lengthwise arrange- 

 ment of the principal organ-systems, which thus lie more or less 

 parallel to one another. This can be observed in invertebrates, 

 such as the annulate worms, in respect of the more nearly original 

 systems. In the vertebrates, the presence of the vertebral column 

 establishes a structural axis, with reference to which the organ- 

 systems are arranged (Figs. 20, 22). 



(2) Metamerism. A large portion of the body, mainly dorso- 

 lateral in position, is arranged on a segmented plan, in which parts 

 are repeated serially and longitudinally around and to either side 

 of the original axis. This segmentation, or metamerism, does not 

 appear to any extent on the surface of the adult body, but becomes 

 evident internally in the subdivision of the vertebral column into 

 vertebrae, and the paired, serial arrangement of the related spinal 

 nerves, vessels, and musculature. Metamerism is externally evident 

 in embryos (Fig. 21, me.) and is founded upon the serial arrange- 

 ment of parts of the mesoderm (Fig. 22, my., d.m.). 



(3) Branchiomerism. This is a secondary segmentation, 

 superposed upon the primary metamerism by the development of 

 a series of gill clefts w^hich do not always correspond precisely with 

 the metameres. It is an adult feature of lower aquatic vertebrates, 

 such as fishes (Fig. 32), where it is expressed in a series of true gill, 

 or branchial structures, associated with gill filaments as functional 



