334 



COMPARATIVE ANATOMY 



gill bars thus formed become interconnected by cross rods or synapticulae 

 such as occur also in urochordates and cephalochordates. 



In urochordates, the number of gill slits varies from a single pair in 

 Appendicularia to the many characteristic of most genera. The gill slits 

 of this group open into an atrial cavity developed by an ectodermal 

 ingrowth along the dorsal side of the body. 



ANTERIOR CAVITY 



MYOTOME 2 



CHORDA 



ENTERON 



MOUT 

 ENDOSTYLE 



LEFT 1ST (TRANSIENT)GILL-POUCH 

 CLUB-SHAPED GLAND 



A. EARLIER LARVA. 



ENDOSTYLE 



RIGHT 1ST PERMANENT GILL-SLIT 



PREORAL PIT 



CLUB-SHAPED GLAND 

 TRANSIENT GILL-SLIT 



LE FT 1ST PERMANENT GILL-SLIT 

 MEDIAN VENTRAL BLOOD-VESSEL 



B.LATER LARVA. 



Fig. 279. — A, young amphioxus larva viewed from the left side as a translucent 

 object redrawn after Hatschek. B, later larva. (Redrawn after van Wijhe.) The 

 mouth of Amphioxus becomes enormously enlarged and, by its growth backward on the 

 left side interferes with the symmetry of development of the gill-slits. The gill-slits 

 of the left side of the body develop before those of the right side. The median line of 

 the ventral side of the pharynx is indicated by the median ventral blood-vessel. Modifi- 

 cation of function and degeneration affect the anterior three pairs of gill-slits. The 

 first pair become the endostyle. The second pair form the transient larval club-shaped 

 gland. The left third slit has no mate and soon disappears. The fourth pair of slits 

 form the first permanent pair. 



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

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

 chordates, the number is doubled by the growth 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 cihary action. The multipUcation of branchial bars means a corre- 

 sponding increase in the number and efficiency of the cilia which cover 



