AMPHIOXUS AND AMMOCCETES. 173 



Ancestral Number of Gill-slits. 



The unlimited number of gill-slits in the adult Amphi- 

 oxus has led to a good deal of controversy as to the ap- 

 proximate number present in the ancestral Vertebrate, 

 some authorities being of the opinion that Amphioxus 

 presents the primitive condition in this respect, and 

 others that the multiplication of gill-slits in this form 

 was a secondary phenomenon. 



Sometimes as many as fourteen pairs of gill-clefts are 

 found in a remarkable cyclostome fish from the Pacific, 

 allied to Myxine, and called Bdellostoma* With this ex- 

 ception, no true fishes, recent or fossil, are known which 

 possess more gill-slits than the existing sharks belonging 

 to the family of the Notidanidce. Of these the genus 

 Hcptanchus possesses eight gill-clefts (i.e. seven plus the 

 spiracle) on each side, and Hexanchus seven. In Ammo- 

 ccetes, as we have seen, there are at one time indications 

 of eight pairs of gill-slits. The first pair of these, how- 

 ever, never breaks through to the exterior, and eventually 

 disappears, but Dohrn has shown that the primary rela- 

 tion in which the seventh pair of cranial nerves stands to 

 it, indicates that it is the homologue of the spiracle of the 

 higher forms. 



Moreover, in the larval development of Amphioxus 

 several facts combine to produce the impression that the 

 indefinite number of gill-slits in the adult is a secondary 

 acquirement. First of all, there is the series of primary 

 gill-slits which, while varying within narrow limits, usually 

 numbers fourteen. Their unpaired unilateral character is 

 merely incidental, as explained above, and it may be stated 



* For a recent account of Bdellostoma, consult HOWARD AYERS, No. 69, 

 bibliography. 



