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ESSENTIALS OF ZOOLOGY 



lower the hypopharyngeal groove, or endbstyle is a struc- 

 ture of great morphological interest, for it is developed in all 

 vertebrates, and in the Craniata it is converted into the 

 thyroid gland. In Amphioxus it is made up of elongated cells, 

 some of which are glandular, some ciliated. The upper or 

 epipharyngeal groove has a similar structure. Between these 

 grooves, which are coextensive with the pharynx, the lateral 

 walls of the pharynx are pierced by numerous gill slits. The 

 gill slits are long narrow openings in the pharyngeal wall, 

 extending obliquely from near the epipharyngeal to near the 

 hypopharyngeal groove, and they open into the atrial cavity. 



Preoral 

 Eye spot pit Velum 



Notochord 



Nerve cord 



Oral cirrus Wheel Velar Periph. band Endostyle Gitt Liver Stomach 



organ tent. 



FIG. 62. Amphioxus. The anterior end of a young specimen. 



The atrial cavity is lined by ectoderm, so that the gills inter- 

 vening between the slits are lined internally by a ciliated 

 endoderm, and externally by ectoderm which is not ciliated. 

 The slits are so numerous and the gills so narrow that the 

 whole system has a very delicate structure. The gills are 

 united by cross-pieces which split up the apertures into several 

 apertures in each case. Moreover, in each gill a thin skeletal 

 gelatinous bar is developed. The bars are connected above the 

 gills, and they end ventrally below the gills, the ends being 

 alternately single and double. The latter are called primary, and 

 the former secondary bars. The regularity of the arrangement 

 is due to the manner of development of the gills. The original 

 slits of the young are divided by a downgrowth into two slits. 

 The primary rods are forked ventrally, but the intervening 

 secondary rods are not forked. Lastly, the pharynx is pro- 

 tected by the walls of the atrial cavity, which not merely 



