198 



ZOOLOGY. 



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but examination of their structure (and still more of their 

 development) shows that, in order to compare them with 

 the dogfish's, the number must first be halved. For each gill- 

 slit originally developed 

 each primary gill-glit 

 becomes divided into two 

 secondary slits by a ver- 

 tical downgrowth. These 

 downgrowths are called 

 the secondary gill-arches 

 and differ in several re- 

 spects from the primary 

 gill-arches that alternate 

 with them. For example, 

 only the primary arches 

 contain a branchial ccelomic 

 canal. (For other differ- 

 ences, see 10.) 



Even when we only 

 count the primary slits 

 we find them greatly in 

 excess of the myomeres in 

 the corresponding region : 

 the metamerism of each of these two sets of structures 

 is independent. Like the myomeres, however, the gill-slits 

 alternate on the two sides, a right primary bar being 

 opposite a left secondary bar. 



From fig. 108 it will be seen that the gill-slits are slightly inclined 

 out of the vertical. It often happens that at death the pharynx is 

 thrown into a distorted shape, the gill-slits coming to lie very 

 obliquely while the endostyle ( 7 below) comes to be convex upwards 

 instead of concave. Hence in prepared transverse sections of the 

 pharyngeal region (as fig. 107), a large number of gill-arches and 

 slits are seen, and the whole pharynx-section appears somewhat 

 heart-shnped instead of narrow-elliptical. 



7. Endostyle. The lattice- work of gill-arches forms the 

 whole surface of the pharynx except for the narrow dorsal 

 and ventral areas, which form continuous grooves 

 dorsally the hyperpJiaryngeal groove, ventrally the endostyle. 

 Anteriorly, in the velum, these two grooves are united by a 



Fig. 107. TRANSVERSE SECTION 

 OF AMPHIOXUS IN PHARYNGEAL REGION. 



(Altered from Marshall and Hurst.) 



