Descriptive Biology HI 



which they were originally called upon to do. In Ampliiuxus 

 and the protocordates (timicates, Balanoglossus) the number 

 of gills is large, occasionally reaching as many as 180 pairs in 

 the former. In the cyclostomes the number varies from six 

 to fourteen while in higher fishes the number is typically 

 five, varying from three to seven. In land vertebrates gills 

 are absent in the adult, but occur more or less developed in 

 the embryo, even man himself at one stage of his existence 

 showing indications of them, a heritage from some remote 

 fish-like ancestor. In their disappearance have these gills 

 left any traces behind them ? There are some undoubted rem- 

 nants of the gills and their associated structures, and others 

 which can be so interpreted only with great doubt. Of the 

 former may be mentioned the spiracle in fishes, an opening 

 from the pharynx to the exterior just back of the head. In 

 land vertebrates this becomes the cavity of the middle ear, 

 which is closed externallv by the tympanum and internally 

 communicates with the pharynx by means of the Eustachian 

 tube. The delicate ear bones, which transmit the vibrations 

 of the tympanum to the inner ear, are probably in part 

 derived from the first gill arch, while from the other arches 

 develop the hyoid bone and some of the laryngeal cartilages. 

 The blood vessels and nerves supplying the gills of the fish 

 are also in some cases directly modified to form other nerves 

 and vessels in the land vertebrate. If then, gill arches have 

 been so markedly reduced in number and can be so profoundly 

 changed as to form parts of the organs of hearing and of 

 speech, why may they not also have been changed to form 

 even more distant parts? Hence the origin of jaws, paired 

 fins and fin supports or girdles have been attributed by some 

 theorists to former gill bars, while even the lungs have been 

 derived, according to one theory, from a pair of gill clefts. 



While there is but little evidence for the latter theories, 

 the former is not an unreasonable one. The position and 

 arrangement of the jaws is such that they can be readily com- 

 pared to a pair of gill arches united below and hinged in 

 the middle to form the upper and lower jaws. Furthermore, 

 the arrangement of the nerves supplying the jaws is very 

 similar to that of those which supply the gill arches. 



For the origin of the paired fins of fishes we have a more 

 likely theory in that of the paired fin fold. The vane of the 

 fish extends discontinuously along the median dorsal line from 

 head to tail, and ventrally is represented by the anal or 

 ventral fins, between tail and anus. Anterior to the anus we 

 find the paired fins occupying varying positions in different 

 fish. According to the paired fin fold theoiy the ancestral 

 fish possessed a continuous median fin, extending dorsally 



