114 Biology in America 



purpose, but this primitive theory has long since been laid 

 to rest. All that we know of the major part of the skull 

 is that as the eyes, ears, nose and brain develop the sur- 

 rounding tissue molds itself to fit them, forming hard parts 

 (cartilage and bone) as a firm support and protection. But 

 there are occasionally found some curious bones at the base 

 of the skull or occiput which strongly suggest that the verte- 

 brae may after all have had something to do in the building 

 of the skull, or at least a part of it. 



Of world-wide distribution, with representatives in South 

 America, Africa, and Australia is a group of fishes known as 

 the lungfishes, which probably represent a connecting link 

 between fishes and Amphibia. In their cartilaginous skeleton, 

 and in their notochord, which persists in the adult, they are 

 very primitive types, while lungs and certain other features 

 mark them as far advanced along the path of evolution. 

 These fishes live in pools which dry up wholly or in part 

 during the dry season and are filled again in times of abun- 

 dant rain. Both lungs and gills appear to be used for res- 

 piration even when the pools are full of water. But when 

 the pools begin to dry up in summer and the water becomes 

 foul with decaying vegetation, the ability to breathe air saves 

 the fish from suffocation. Some species during the dry season 

 settle comfortably into the mud, retaining communication 

 with the outer world by means of a hole in the mud, at the 

 bottom of which they lie. Here they breathe air, resuming 

 the gill habit when the rainy season once more replenishes 

 their pools. In these fishes there is a ' * cranial rib ' ' attached 

 to the base of the skull somewhat resembling the true ribs 

 of the fish and suggesting that a vertebra bearing this rib 

 has been united with the skull. 



The cyclostome skull forms but an incomplete, basket-like 

 frame work for the brain and does not extend behind the ear, 

 leaving the ninth and tenth nerves outside, which in higher 

 fishes become enclosed in the skull. When we come to the 

 land vertebrates, the reptiles, birds and mammals, this process 

 of inclusion of nerves within the skull goes a step further 

 and we find twelve instead of ten nerves issuing from the 

 skull. While this process of telescoping as it were the head 

 end of the animal is going on many of the nerves and muscles 

 are being crowded out, while others are so modified as to bear 

 but little resemblance to their former selves. Evidence of 

 the loss of nerves and muscles in the evolution of the verte- 

 brate head may be found in the presence of more numerous 

 nerves and muscles in this region in adult cyclostomes than 

 are found in the adults of the jawed vertebrates, and espe- 

 cially in the development of the latter, where a varying num- 



