THE RESPIRATORY SYSTEM 



2!; I 



Amphioxus, the typical genus of cephalochordates, has as many as 

 one hundred and eighty paired openings or stigmata. As in hemichor- 

 dates, the original number is doubled by formation of secondary gill-slits. 

 Before metamorphosis the number of primary gill-slits in the larva of 

 Amphioxus is nineteen pairs. The large number of gill-slits in the proto- 

 chordates is apparently an adaptation, since these organisms use their 

 gills not only for respiration but also as a mechanism for obtaining food 

 by ciliary action. 



The history of gills in vertebrates is one of continuous reduction in 

 number and modification in function. The transformation of their 

 skeletal supports has been described in the history of the skeletal system. 

 Even in Amphioxus, some of the gill pouches of the embryo are modified 

 or lost. (Fig. 233) The first pair become the endostyle while the second 

 pair form the larval club-shaped gland. The third left slit never has a 

 corresponding right slit and disappears early in ontogenesis. The first 



Fig. 234. — Diagram of relations of esophagus and respiratory tracts in (A) Myxine 

 and Ammocoetes, and {B) Petromyzon; b, branchial duct ("bronchus"); oe, esophagus; 

 t, thyroid gland. (From Kingsley's " Comparative Anatomy of Vertebrates.") 



permanent gill-slit of Amphioxus is therefore the fourth of the ontogenetic 

 series. There are cogent reasons for homologizing this sHt with the 

 spiracle of elasmobranchs, but there is httle agreement among morpholo- 

 gists in regard to the exact homology of serial organs in chordates. 



The popular belief among morphologists that the vertebrate mouth 

 has been formed by the coalescence of a pair of gill-sHts is supported by 

 the mode of development of the mouth in Amphioxus. The endoderm 

 takes the initiative in the development of the mouth of Amphioxus, as 

 would be the case if it were a gill-slit. In this respect, the mouth of 

 Amphioxus differs from that of vertebrates, in which the ectoderm initiates 

 development. 



The question whether or not gills are metamenc structures nas been 

 an open one. The metamerism of chordates is manifested primarily in 

 the mesodermal somites. Since there are none of these in hemichordates 

 and urochordates, it is impossible to demonstrate in these forms a cor- 

 respondence between mesomerism and branchiomerism, and thus to 

 estabHsh the metamerism of the latter. The case is different, however, 

 in Amphioxus, where the mesodermal segmentation is one of the most 

 striking features. In the adult animal, there is no correspondence 



