174 MEKISTIC VARIATION. [part i. 



141. B. polytrema : single specimen from Chili, badly preserved 

 but apparently having fourteen pairs of gill-openings. GuNTHEJt 

 I. c, p. 512. 



Specimen having 14 gill-openings on left side and 13 on right. 

 Sciinkii.ki:. A., Arch. f. iWaturg., xnvi. 1S80, p. 115 (cp. Putnam, 

 Proc. Bost X. 11. &, xvi. L873, p. 1G0). 



/;. bischoffii: single specimen, 10 gill-openings on each side. 

 ibid. 



Ammoccetes : having eighl branchial openings on each side instead 

 of Beven, the normal number. The shape of the mouth of this specimen 

 was also abnormal, being described as somewhat square. [No satis- 

 factory description.] Eowabo, Thomas, Zoologist, xvi., p. 60D7. 



142. In connexion with individual Variation in the number of gill- 

 sacs in Myxinoids it should be borne in mind that in Petro- 

 myzon there are normally seven pairs of gill-sacs. The case of the 

 Notidanidse may also be mentioned in this connexion. Among 

 Selachians the Notidanidse are peculiar in having a number of gill- 

 slits other than rive, and of them liexanchits has six pairs, while 

 Heptanchus has seven 1 . 



III. Cervical Fistula and Supernumerary Auricles ix 

 Mammals. 



Though the evidence of this subject is well known and has 

 often been collected, it may be convenient to give here some 

 abstract of the facts in so far as the phenomena of Variation are 

 illustrated by them. Since cervical fistula? have been believed to 

 result from the persistence of the embryonic branchial clefts, they 

 may properly be considered in relation to the general question of 

 Variation in the number of gill-slits, while the development of 

 external appendages, perhaps serially homologous with the external 

 ears, directly concerns the subject of Meristic Variation. 

 Man. The subject has been studied by many observers, espe- 

 cially by Aschersox 2 , and by Heusixger 3 , who brought together 

 and abstracted 4(i cases, being all that had been described in Man 

 up to LS(j4. G. Fischer 4 gives a full list of the literature of the 

 subject up to 1870, with an analysis of 65 cases. A further paper 

 by Heusixger 5 contains a general account of these structures as 

 they are found in Man and in the domestic animals. Additional 

 cases, together with a general discussion of the subject, especially 

 in relation to fistula? on the external ears, were given by Sir James 



1 Balanoglossus. In five species with which I am acquainted, the number 

 of pill-bars and slits varies in proportion to the size of the body, and as it is not 

 unlikely that these animals continue to grow throughout life, it is probable that the 

 number of branchiae is always increasing by formation of new gill-slits at the 

 posterior end of the branchial region. The same is probably true of Amphioxus. 



2 Ascherson, BefistulU colli congenitis, Berlin, 1832. 



3 Heusinger, Anh. f. Path. Anat. u. Phys., 1864, xxix. 



4 Fischer, G., Dent. Ztsch.f. Chirurg., 1873. 

 8 Heusinger, Deut. Ztsch. f. Thierm., 1878. 



