THE LAMPREY. 19 



posteriorly with the intestine in the antero-dorsal region of 

 the body-cavity. Its diameter is not more than half that of 

 the respiratory tube, and its walls are very delicate and 

 easily torn. The best way to make it out without damage 

 is to pass a guarded bristle into it from the already opened 

 intestine. 



60. The pericardium, a large chamber, lying immedi- 

 ately behind the last pair of gill-sacs : its posterior wall, as 

 already seen ( 44), is strongly convex ; its anterior surface 

 fits closely against the hinder or inner walls of the last pair 

 of branchial sacs, and presents therefore a double concavity. 

 In the present view, of course, only the left of these con- 

 cavities is seen. 



r 



61. The circum-cesophageal sinus (Figs. 4 and 6, s), a consider- 

 able longitudinal cavity, lying above and at the sides of the gullet, and 

 imperfectly divided into two tubes by a perforated vertical partition which 

 is related to the gullet like a mesentery. Behind the posterior end of the 

 respiratory tube, the sinus extends also below the gullet as far downwards 

 as the ventral aorta ( 72) and between the inner or posterior walls of the 

 last pair of gill-sacs, this portion (Fig. 4, /) being also divided by a per- 

 forated vertical partition. ,The cavity is related to the gullet in much 

 the same way as the body-cavity to the intestine. 



XI. If the anterior (pre-branchial) portion of the ven- 

 tral body-muscle ( 34) is still left, remove it, and 

 note 



62. A great mass of muscle, forming a prominent pro- 

 jection beneath the eye, and just in front of the gills : it 

 forms the lateral boundary of the mouth-cavity and contains 

 imbedded in it the subocular arch ( 7), styliform process 

 ( 8), and cornual cartilage ( 9). 



63. The salivary sac, appearing as a dark patch on the ventro- 

 lateral region of the above mass of muscle : when cut into, it is seen to 

 be a cavity with glandular walls : it is said to open by a fine duct 

 into the mouth. 



C 2 



