THE RESPIRATORY SYSTEM 



333 



History of the Gills. Pharyngeal gills are peculiar to chordates and 

 are one of the most constant characteristics of the group. This should 

 not be understood to imply that invertebrates are without structures 

 from which gills might have evolved. The origin of gills from endodermic 

 diverticula suggests the possibility that their beginnings may be seen in 



NEUROPORE BRAIN 

 MOUTH t -I . 



SPINAL CORD 



NOTOCHORD 



ENCXX>ERMIC STRAND 



A. LARVAL UROCHORDATE. 



NEUROPORE MYOTOME I 



NOTOCHORD 



NEURENTERIC CANAL 



M 

 ENDOSTYLE 

 CLUB-SHAPED GLAND 



B. LARVAL AMPHIOXUS. 



ENTERON 

 STCTRANSIENT) CILL-SUT 



ANTERIOR CAVITY 

 NEUROPORE 



NOTOCHORD 

 BLASTOPORE 



NEUROPORE 



hfYOTOME. I 



NEURAL TUBE 



NOTOCHORD 



ENDOSTYLE 

 CLUB-SHAPED GLAND 



ENTERON 



C. AMPHIOXUS EMBRYO. 



ANTERIOR . 

 CAVITY 

 ENDOSTYLE' 

 CLUB-SHAPED GLAND 



1ST (TRANSIENT) GILL-SLIT 



D. AMPHIOXUS OLDER EMBRYO. 

 Fig. 278. — While gill-slits occur both in urochordates and in cephalochordates 

 (Amphioxus), evidence is lacking that the anteriormost gill-slits of the two groups can 

 behomologized. Those of the urochordate are permanent, while those of amphioxus 

 do not persist as functional gills. The first pair of pouches in amphioxus become the 

 endostyle, the second pair form the transient larval club-shaped gland, the left third gill 

 pouch disappears early in ontogenesis. As shown in D the anterior endodermic diver- 

 ticula (anterior cavities) resemble gill pouches in relations and in mode of origin. 



the intestinal diverticula of flatworms. Were these diverticula to meet 

 the skin and become perforate, apertures similar to gill sUts would be 

 formed. 



Gill slits first appear in the hemichordates. Rhabdopleura has none, 

 but cephalodiscus has a single pair. In most hemichordates the number 

 is considerable and increases throughout Hfe. Early in their development, 

 their number is doubled by the growth of "tongue-bars" which extend 

 from the dorsal side of the gill aperture to the ventral side. Later the 



