274 THE VARIATION, llACES AND MIGRATIONS OF THE MACKEREL. 



Table IX., showing freqiiency of finray numhers in small and large 

 Plymouth fish, irrespective of sex. 



Small fish = below 10^ inches; minimum size 5;V inches. 

 Large fish = above 12| inches; maximum size 15| inches. 



As regards the cause of the reduction in the number of recognisable 

 finrays, there are two possibilities, (1) the gradual absorption, and (2) 

 the gradual concealment of the minute posterior rays. 



I have been unable as yet to determine the extent to which actual 

 absorption of the rays takes place, but I believe that some part of the 

 reduction, if not the whole of it, is due to changes in the relation 

 of the fin to the neighbouring tissues during growth of the fish. In 

 an adult mackerel the first dorsal fin is lodged in a deep groove, within 

 which it can be entirely depressed — an adaptation, without doubt, to 

 habits of rapid locomotion, as this fin is never used for swimming, and 

 would only be a hindrance if incapable of being bent back and tucked 

 away within its socket ; but in young fish up to 7 or 8 inches in 

 length the groove is not yet formed. Consequently, in old mackerel 

 the basal part of the fin is sunk beneath the general surface of the 

 skin, while in young mackerel every part of the fin is freely exposed. 

 The minute posterior rays do not protrude outside the groove in old 

 fish, but are clearly visible in young fish. In these they require no 

 preparation or dissection to be displayed ; but in old fish the groove 

 has to be carefully explored with a seeker, and often the lateral flaps of 

 skin have to be cut away for the purpose, before the number of 

 projecting rays can be accurately ascertained. This, of course, is a 

 mere matter of care, which it is needless to say was invariably 

 bestowed in the course of the investigation, the number of rays in 

 this fin having been counted in each fish at least twice, and often four 

 or five times, before being recorded. But a real difference between 

 the two conditions consists in the fact that the posterior rays in large 

 fish generally project less above the floor of the groove than do 

 the corresponding rays of small fish, thus indicating in large fish a 



