Nos. IAND2.] ANATOMY OF SCOMBER SCOMBER. 165 



in those so-called double systems that were formed where inde- 

 pendent main canals had fused with each other. 



The descriptive terms relating- to the canals, adopted, mainly 

 from earlier writers, in my works on Auiia, are still retained, with 

 but few slight changes, in the present descriptions, for, until the 

 anatomy and development of the lateral system, and of the nerves 

 related to it, are much better and more fully known than at 

 present, I see no advantage whatever to be gained in changing 

 them. I may, however, here state that what I have called the 

 main infraorbital canal of the head is certainly formed by the 

 fusion of several more or less independent canals, which should, 

 in all probability, be carefully distinguished one from the other. 

 This will be more especially referred to in describing the canal. 



The single tubes that lead primarily from a main canal, be- 

 tween two successive organs of that canal, to the outer surface of 

 the head, I here call primary tubes or trunks, as in my earlier 

 work. Where there are several such tubes resulting from the 

 subdivision of a single primary tube, or a number of branch 

 canals, arising from a single trunk, they are called, collectively, 

 a peripheral canal system. These peripheral systems, and the 

 pores that form their surface openings, always arise, so far as my 

 experience goes, by the repeated dichotomous division of a single 

 primary pore and tube ; by the fusion and subsequent dichotomous 

 division of two such pores and tubes, one belonging to one canal 

 and the other to another; or by a similar fusion and subsequent 

 division of certain parts of two adjoining peripheral systems. In 

 each of the two latter cases I have called the resulting system a 

 double system. The systems at the ends of a principal canal are 

 called terminal systems, or terminal tubes and pores. 



• 

 I. Main Infraorbital Canal. 



The main infraorbital canal of Scomber begins, anteriorly, at a 

 surface pore which lies considerably below and a little behind the 

 single nasal aperture of the fish, and a little in front of the anterior 

 edge of the gelatine-like substance that surrounds and partly 

 covers the eye. The canal leading inward from this pore enters 

 the lachrymal near its antero-dorsal edge, and runs backward and 



