CHAP. VII.] 



CYCLOSTOMT. 



173 



ing from the oesophagus and a separate aortic arch supplying it. 

 On the right side the sixth and seventh pouches were practically 



* 



r D asu 



Fig. 27- Myxine glutinosa ; specimen having seven pairs of branchial sac-. 

 Diagram shewing branchial sacs, heart and aortic arches from the dorsal surface. 

 On the right side the sixth and seventh branchial sacs were partially confluent. 



D. oes., ductus cesopJiageus. 



(From a drawing kindly lent by Prof. Weldon.) 



confluent though each had a separate oesophageal opening and a 

 separate arch from the aorta. In the drawing, for which I am also 

 indebted to Professor Weldon, the oesophageal openings are not 

 shewn. 

 40. Bdellostoma. In this genus the number of branchial sacs is 

 variable, different numbers being found in different species and 

 individual variation also occurring. 



The generic name Heptatrema was originally given by Dum£ril 

 from the presence of seven gill-sacs. In 1834 Joh. MtJLLER, finding 

 that this character is not constant proposed the name Bdellostoma. 

 Of three Cape specimens examined by him one had seven gill-sacs 

 on each side, one had six on each side, and one had six on the right 

 side and seven on the left. To these he gave the names B. hepta- 

 trema, B. hexatrema and B. heterotrema respectively (Abh. k. Ah 

 Wiss. Berlin, 1834, pp. 66, 67 and 79, Taf. VII.). Further observa- 

 tion has shewn that the number of gill-sacs in the Cape Bdello- 

 stoma is liable to individual variation, some specimens having six 

 while others have seven. The name B. cirrhatum (GtJNTHER, Gat. 

 Brit Mus., viii. 1870, p. 511) includes these and the New Zealand 

 specimens. As to the relative frequency of specimens with six or 

 seven pairs or with an asymmetrical arrangement I have no in- 

 formation. A collection lately brought from the Cape by Sedgwick 

 includes one specimen with six pairs and several with seven pairs. 



