THE LOWER VEKTEBRATA 175 



called the gill-arches (or branchial arches). They are 

 strengthened internally by bars of cartilage, and from them 

 arise the actual gill-processes in which the exchange of gases 

 between blood and water is effected. These processes are 

 very soft folds of the mucous membrane of the pharynx, 

 covered with a thin epithelium, and containing a core of 

 highly vascular connective tissue. The whole series of gill- 

 processes on one side of a gill-arch is called a demibranch 

 (i.e. half -gill). There are on each side of the pharnyx 

 nine functional demibranchs, and a vestigial one on the 

 front face of the spiracle. The last branchial arch (the 

 one posterior to the last gill-cleft) bears no gill-processes. 

 The aeration of the blood in the gill-processes is effected 

 by the suction of water into the pharynx and its- expulsion 

 through the gill-slits. The muscular mechanism is roughly 

 similar to that seen in the frog ; alternate enlargement 

 and diminution of the capacity of the pharynx are caused 

 by the contraction of two sets of muscles attached to 

 ventral plates of cartilage (basi-hyal and basi-branchial) 

 which answer to the frog's hyoid plate. In inspiration 

 the mouth opens, the pharynx fills with water, and the 

 mouth closes. In expiration the water is forced out 

 through the gill-slits and over the gill-processes, the 

 branchial arches separate widely by contraction of their 

 muscles, and the oesophagus closes by contraction of its 

 muscular wall. The water passing over the gills is 

 separated from the blood by a thin membrane, through which 

 an interchange of dissolved gases takes place oxygen 

 from water to blood, carbon dioxide from blood to water. 



8. The Heart. The circulation is entirely devoid of 

 that double character which is rather imperfectly attained 

 in the adult frog, and perfectly in the rabbit. There being 

 no lungs, there are no pulmonary veins, and no division 

 even of the auricle into two, still less of the ventricle. All 

 the blood that passes through the heart is deoxygenated 

 and has but one possible course through sinus venosus, 

 auricle, ventricle and truncus arteriosus. In fig. 91, a 

 comparison of the heart of dogfish and frog is given. It 

 will be seen from it that the condition in the frog is arrived 



