66 Evolution and Adaptation 



have added to the cartilaginous skull certain plates in the 

 dermal layer of the skin. In the higher forms we find the 

 skull composed of two sets of bones, one set developing from 

 the cartilage of the first-formed cranium, and the other having 

 a more superficial origin ; the latter are called the membrane 

 bones, and are supposed to correspond to the dermal plates 

 of the ganoids. 



In the development of the kidneys, or nephridia, we find, 

 perhaps, another parallel, although, owing to recent dis- 

 coveries, we must be very cautious in our interpretation. As 

 yet, nothing corresponding to the nephridia of amphioxus has 

 been discovered in the other vertebrates. Our comparison 

 must begin, therefore, higher up in the series. In the sharks 

 and bony fishes the nephridia lie at the anterior end of the 

 body-cavity. In the amphibia there is present in the 

 young tadpole a pair of nephridial organs, the head-kidneys, 

 also in the anterior end of the body-cavity. Later these are 

 replaced by another organ, the permanent mid-kidney, that 

 develops behind the head-kidney. In reptiles, birds, and 

 mammals a third nephridial organ, the hind-kidney, develops 

 later than and posterior to the mid-kidney, and becomes the 

 permanent organ of excretion. Thus in the development of 

 the nephridial system in the higher forms we find the same 

 sequence, more or less, that is found in the series of adult 

 forms mentioned above. The anterior end of the kidney 

 develops first, then the middle part, and then the most poste- 

 rior. The anterior part disappears in the amphibians, the 

 anterior and the middle parts in the birds and mammals, so 

 that in the latter groups the permanent kidney is the hind- 

 kidney alone. 



The formation of the heart is supposed to offer certain 

 parallels. Amphioxus is without a definite heart, but there is 

 a ventral blood vessel beneath the pharynx, which sends blood 

 to the gill-system. This blood vessel corresponds in position 

 to the heart of other vertebrates. In sharks we find a thick- 



