242 CHARACTERS OF VERTEBRATE ANIMALS 



bar-like thickening or gill-arch. These bars and slits are good 

 examples of visceral arches and clefts, the presence of which at 

 some period of life or other is one of the primary characteristics 

 of a Vertebrate (see p. 62). The three gills on each side grow 

 out from the tops of the three first gill -arches. The heart of 

 the larva contains impure blood only, returned from all parts 

 of the body, and its function is to pump this blood to the gills 

 for purification through paired afferent branchial vessels which 

 run within the gill-arches. The purified blood is collected up 

 from the gills and distributed to the body by efferent branchial 

 vessels which unite above to form a dorsal aorta. Each afferent 

 vessel, with the corresponding efferent one, constitutes what may 

 be called an aortic arch. As the larva gradually assumes the 

 structure of the adult all the gill-slits close, the gills at the 

 same time shrivelling up. Meanwhile the lungs have increased 

 in size, and take on the work of purifying the blood, and the 

 aquatic gill-bearing larva is thus converted into a terrestrial air- 

 breathing Salamander. The heart now receives not only impure 

 blood from the body, but also purified blood from the lungs, 

 and it becomes necessary to solve the problem of how to keep 

 these two sorts of blood separate by modifying an arrangement 

 specially adapted for pumping impure blood to the gills. To 

 use an illustration, it is as if a pumping apparatus made for 

 distributing cold water were to be also connected with a warm- 

 water supply, and one were then called upon to modify the 

 apparatus so as to keep the two kinds of water as distinct as 

 possible. The problem is only partially solved in the Salamander, 

 for the separation of the two kinds of blood is incomplete, the 

 result being that some of the blood distributed by the heart is 

 impure, some pure, and the rest mixed. Just as if, in modifying 

 the supposed pumping apparatus, we succeeded to a certain 

 extent, part of the warm and cold supplies mixing, however, to 

 give tepid water. In such an event the pump would distribute 

 three kinds of water as regards temperature, i.e. cold, warm, and 

 tepid, these corresponding to the impure, pure, and mixed blood 

 of the Salamander. 



It would take too much space to fully describe how the 

 circulatory organs of the Salamander are modified during the 

 change from larva to adult, but some of the leading features 

 may be noted. To begin with, the originally single auricle is 



