SPIRAL VALVE IN THE GENUS EAIA. 



55 



possessed by the intestine. I am enabled to express these differences numerically, 

 through the kindness of my friend Mr. Ambrose R. Willis, B.Sc, who has taken the 

 trouble to make for me the necessary calculations. 



Taking the intestine represented in fig. 1, PI. X., as the nearest approach to type A, 

 and for S, C, and D, figs. 4 (PI. X.), 1 & 3 (PI. XI.) respectively, we have the follow- 

 ing dimensions : — 



In this form, however, the areas, which it must be understood are only approximately 

 correct, are not readily comparable, owing to the different sizes of the four specimens ; 

 and it is convenient to take a common length both for the intestine itself and for the 

 attached edge of the valve. Taking as a standard these dimensions as they occur in A, 

 in the above Table, we have, for all four types, the internal area of the intestine equal 

 to 44-3 sq. cm., and the other areas as follow : — 



Thus, in intestines of the same size, a valve of the type I) will have about two and a half 

 times the area of one of the type A, while the entire absorption-surface {i. e. area of 

 both sides of valve plus internal area of intestine) will be as nearly as possible twice as 

 great in the former as in the latter — a fact as remarkable as if (the average length of 

 the human intestine being 25 feet) subjects were occasionally found with only 20 feet, 

 and others with as much as 40 feet. 



§ 10. The .relation, in respect of absorption-surface, between the four types is stri- 

 YOii. XI. — PART II. No. 6. — August, 1880. k 



