CHAPTER VII. 



linear series — continued. 



BRANCHIAL OPENINGS OF CHORDATA AND STRUCTURES IN 



CONNEXION WITH THEM. 



Under the general heading of Variation of branchial openings 

 facts will be given relating to the following subjects. 



I. Variation in the patterns formed by the bars, vessels and 

 stigmata of the branchial sac in Ascidians. 



II. Variation in the number of gill-sacs in Cyclostomi. 



III. Abnormal openings in the cervical region of Mammals, 

 known as " cervical fistula 3 ," and external appendages called " cer- 

 vical auricles," or " supernumerary ears," present sometimes in 

 connexion with such openings. 



With reference to the two first subjects the evidence is only 

 fragmentary, but the instances recorded seem to be of sufficient 

 consequence to warrant their introduction in illustration especially 

 of the magnitude and definiteness of Variation. 



Variations affecting the opercular opening in Amphibia are mentioned in 

 connexion with Bilateral Series. 



I. Ascidians. 



Transverse vessels of Branchial Sac. 



132. Ascidia scabra. Branchial sac in one specimen shewing abnormal 

 and irregular structure owing to branching of transverse vessels. The 

 resulting appearance is entirely peculiar. Herdmax, W. A., J. Linn. 

 Soc. (Zool.), 1881, xv., p. 284, PI. xvii., fig. 3; also p. 330. 



133. Ascidia virginea (O. F. Miiller): a case of great irregularity 

 exactly similar to the above. Ibid., p. 330. 



134. Ctenicella lanceplani. Branchial sac may present characters due 

 to variations in disposition of transverse vessels etc., which assume three 

 distinct patterns or marked varieties. Lacaze-Duthieks, Arch, Zool. 

 Exp., S. 1, Vol. vi., p. 619, Vol. xxxiii., rigs. 9 — 11. 



