SPIEAL VALVE IN THE GENUS EAIA. 57 



very perfect "columella" exists, and, owing to the peculiar form of the turns, the food 

 must be continually forced into the narrow ends of the " cones " (that is, in a retrograde 

 direction), the obstruction to its rapid passage being far greater than even in the last 

 case. 



§ 12. The last of the variable features mentioned in the Table on p. 51 is the cha- 

 racter of the mucous membrane. How this differs in different individuals will be seen 

 from the enlarged surface-views given in PI. X. figs. 10-13. The membrane may be 

 raised into a strong network of ridges, both ridges and intervening spaces being covered 

 with papillae ; or there may be a very open network with a fine hexagonal reticulation 

 in the intervals ; or the fine reticulation only may be present, and no well-marked 

 ridges deve]oped\ 



It is also a fact worth mentioning, that when the mucous membrane exhibits strong 

 papillose ridges it is usually, in chromic-acid specimens, hard and rough to the touch, 

 almost like the rumen or reticulum of a ruminant ; the whole wall also is very thick 

 and resistant. On the other hand when the membrane appears finely pitted owing to 

 the delicate reticulations on its surface, it feels soft, and the whole wall is usually more 

 or less flabby. 



§ 13. All these things taken together — the variation in the extent of the absorption- 

 surface, in the resistance to the passage of food, and in the characters of the mucous 

 membrane — make the individual differences in the intestine very great indeed. Alto- 

 gether I am inclined to think that this is the most remarkable case on record of spon- 

 taneous vai-iation in nature, since the variable structure is neither a rudimentary, nor a 

 comparatively useless, nor a merely ornamental one, but is one the perfection of which 

 is of the highest importance to the animal's well-being. It is quite true that, as Pro- 

 fessor Huxley suggested to me, the Skate with the improperly developed valve has 

 nothing to do but to eat more ; but eating more means catching more ; and catching 

 more entails a greater expenditure of energy ; so that, in any case, the animal in ques- 

 tion must be rather heavily handicapped in the struggle for existence. 



§ 14. I have examined the intestines of a few other Elasmobranch genera, and now 

 give the result of my observations. These must be taken for what they are worth, 

 which is, perhaps, not much ; for with the case of the Skate before us, it seems of little 

 use to record any but an extensive series of observations. 



In a large specimen oiScyllium canicula I find an especially interesting form of valve, 

 one, in fact, which showed a far higher development of the type B than any I had 

 found in the Eay. This intestine is shown in fig. 5, PI. XI. : there are twelve turns to 

 the valve, all but the last of which are strongly defiected forwards, producing a struc- 



' The specimens brought from the market are rarely in a sufficiently fresh condition to repay microscopical 

 examination ; but if a large number of fresh specimens could be obtained, I am convinced that the histology 

 of the Bay's intestine would be well worth careful study. 



£2 



