!/> 



304 PROFESSOE, FRANK J. COLE 



the others, and more ventral. It has its own afferent artery quite distinct from the 

 one in front. Gills normal on the right side. 



(g) Gills normal on both sides, but only five afi'erent branchials arose from the 

 aorta. The last, however, on each side split almost immediately to supply the last 

 two gills. 



(h) Seven gills on the left side. — Other conditions exactly as in (c). 

 {i) Seven gills on the left side. — Other conditions as in (c). 



It thus appears that the usual variation is the presence of an extra gill on the left 



side, the ductus oesophago-cutaneus being still present and opening independently. 



n fe '^^^^ ^^^^ described by Howes is one such, but the one mentioned by Bateson had seven 



J"*^ j^ ^^^ pairs of gills and a ductus. Howes considers that the extra left pouch is the ductus 



^ ' oesophago-cutaneus converted into a gill, so that the new feature in the varying animal 



Jl ■ is the ductus. This, of course, is quite possible, although it does not explain the additional 



Ir^ gill on the right side, where no ductus has ever been found. We have therefore in 



Myxine the potential existence of a pair of gills behind the sixth + the ductus, 



which may conceivably represent an eighth pair. 



E. The Mucous Surfaces. 



These are described from behind forwards, and as if the gut were split up along 

 the longitudinal plane. 



The abdominal intestine is a typical mid-gut, and is a straight and perfectly 

 uniform tube in which regions cannot be distinguished either macro- or microscopically. 

 It has a maximum width of 11 mm. in a 24-cm. Hag, and is moored to the roof of the 

 abdominal cavity by a median dorsal mesentery (cp. Part II., fig. l). Over the gut 

 dorsally, and slightly to the right, are the gonad and the portal vein, and also the 

 so-called sympathetic nerve. The latter is the most median of the three ; then comes 

 the vein, and finally the gonad, which lies at first over and then laterally to the 

 vein, as shown in the above figure. The walls of the vein are very thin, and the 

 vessel is seen rather by its contents than by its walls. At more or less regular 

 intervals the arterial supply is seen to pass on to the gut via the mesentery. 



The mesentery is easily detachable from the mid-gut in preserved animals, and 

 the gut itself, where its form is not distorted by the development of the ovarian 

 portion of the gonad, and in front by the liver, is smooth, and almost spherical in 

 transverse section. Its outer wall exhibits an elaborate pattern due to the ramification 

 of the very extensive vascular supply of its curious lymphoid coat. This pattern 

 characterises practico,lly the whole length of the abdominal gut, but it is somewhat 

 simplified for about 6 mm. in front of the anus — in other words, the lymphoid tissue 

 is wanting in the region of the hind-gut, and it simplifies again gradually in the 

 neighbourhood of the posterior lobe of the liver, and may be lost altogether about 

 10 mm. behind the opening of the bile duct. 



