Page:Proceedings of the Royal Society of London Vol 69.djvu/23

Rh rostral portion of the fin, it has not been thought necessary to con- sider the two sexes separately on this point. The following table shows the way in which the variations in the position of the first girdle-piercing nerve are associated with the variations in the number of the collector nerves.

Table VIII.

1st girdle- piercing nerve.

Collector nerves.

Total.

Average.

9.

10.

11.

12. | 13.

35

10 6 2

8

4 19 20 8 3

8 26 23 21

1

8 1 11 1 11 1

i : -

23 60

57 49

10 -00 10 '65 10-80 10 -79 11 /?,? 12-00

36

37

38

39

40

From this table it will be at once seen that the more rostnil position of the girdle is on the whole correlated with a reduction in the number of nerves taking part in the formation of the nervus collector. The only break in the ascending series of the average number of collector nerves as the girdle becomes more caudal, occurs where the girdle is pierced by the thirty-seventh and thirty-eighth nerves.

Here the average number of collector branches is practically identical in the two cases. The criticism may be made that a number of old embryos are included in the above table, and that, if we may judge from the case of Mustelus ((12), p. 335), these may show a larger number of collector branches than the adults, and so tend to bring irregularities into it. Such a criticism may be disposed of by the following two tables, in which a comparison is made between such embryos and the adults.

Table IX.

No. of collector nerves. . 9.

i

10.

11.

12.

13.

Arerat/e.

Embryos 6

23

26

6

10 -5<1

Adults 20

31

52

33

2

10 '7T>

Table X.

First girdle-piercing nerve.

35.

36.

37.

38.

39.

40.

Average.

Embryos

7

23

18

32

1

1

36 '67

Adults

16

37

39

37

8

1

36 -Ql