152 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



Later on, the first and some of the posterior ones close up, while 

 the others gradually shift round across the mid-ventral line to 

 the left-hand side of the body and become the anterior slits of 

 that side. 



(2) Secondary Slits 7, 8, or 9 in number appear on the 

 right-hand side of the body above the primary ones, the first slit 

 corresponding with the gap between the 2nd and 3rd primaries. 

 So many of the primaries close up as is necessary to make their 

 number equal to that of the secondaries. Thus if there are 8 of 

 these last, 6 of the primaries close, as follows, using Roman 

 numerals for the primaries, arabic numerals for the secondaries, 

 and brackets to indicate closure : 



87 654321 



[XIV] [XIII] [XII] [XI] [X] IX VIII VII VI V IV III II [I] 

 The secondary slits become the anterior slits of the right side. 



(3) Tertiary Slits are formed on each side behind those already 

 developed. New ones are added as long as the animal continues 

 to grow in length. 



Primaries, secondaries, and tertiaries alike become divided into 

 two by the downgrowth of secondary bars, as described on p. 141. 



A trial Cavity. The primary gill-slits at first open directly to 

 the exterior, but later on into a groove which is bounded by a, 

 ridge on each side. The groove is then converted into a narrow 

 canal, open at both ends, by the fusion of two shelf-like subatrial 

 folds which grow out from the ridges. This canal, which becomes 

 the atrial cavity, sinks as it were into the body, gradually extend- 

 ing round the gut so that the new gill-slits open into it. The 

 anterior opening soon closes, but the posterior one remains as the 

 atriopore. 



CHAPTER IX. PISCES (Fishes). 



THE lowest group of Vertebrates, using that term in the more 

 restricted sense (see p. 135), is constituted by Fishes, which are 

 again arranged in smaller subdivisions, as follows : 



