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: 
1. The formation of an axial skeletal support, consisting 
primarily of a strand of cellular tissue, the notochord, and second- 
arily of a segmented cartilaginous, afterwards bony, vertebral 
column. 
