48 Anatomy of the Rabbit. 



succession of true gill or branchial structures, which support gill 

 filaments as functional respiratory organs. In higher terrestrial 

 animals it appears as an embryonic feature (Fig. 22, m.h. 1,2) and 

 is to be considered both as a determinant of adult form and as a 

 mark of aquatic ancestry. As in lower vertebrates, it underlies not 

 only structures of branchial significance (branchial arches in the 

 restricted sense, but also modified branchial structures, including 

 the first or mandibular arch (m.), and the second or hyoid arch (h.). 

 So great is the modification of these structures in passing from the 

 embryonic to the adult condition that the recognition of the ground- 

 plan is perhaps here of less general importance. It, however, 

 determines the position and relations of certain skeletal structures, 

 including the auditory ossicles, the hyoid, and in part the laryngeal 

 cartilages — a point of some value in the classification of the parts 

 of the head skeleton. It also determines the succession of certaia 

 soft structures, including the fifth, seventh, ninth and tenth cranial 

 nerves; also the chief arterial vessels of the heart, which are more 

 fully referred to below. 



The fundamental importance of branchiomerism lies in the 

 fact that respiration by means of gill perforations of the pharynx is 

 characteristic of that branch of the animal kingdom from which all 

 chordate animals have been derived. In the various invertebrate 

 phyla are found respiratory surfaces of many kinds, in which, 

 however, thin surface membranes, external tufted, or invaginated 

 tubes and analogous structures, provide for diffusion without 

 perforation of the body tube. 



EMBRYONIC PLAN OF THE SYSTEMS. 



In the individual organ-systems the main features of the general 

 plan, as estimated on embryonic development, may be outlined as 

 follows : 



I. The formation of an axial skeletal support, consisting 

 primarily of a strand of cellular tissue, the notochord, and second- 

 arily of a segmenteoTcartilaginpus, afterwards bony, vertebral 

 column. 



