Page:Proceedings of the Royal Society of London Vol 69.djvu/16

8 The range of variation with regard to the following points will be first examined, after which it is proposed to find out whether any correlations between the different parts can be established.

I. The serial number of the first girdle-piercing nerve.

II. The number of the post-girdle nerves.

III. The number of nerves forming the collector.

IV. Asymmetrical variations.

V. The number and position of the nerve canals.

VI. The number of the fin rays.

VII. The number of the whole-vertebrse.

I. Variations in the Serial Number of the First Girdle-piercing Nerve.

According to Davidoff's account the pelvic girdle is pierced by two foramina, through which pass the ventral elements of two consecutive fin nerves. Of these two foramina the more laterally situated is the larger, and affords a passage to the ventral elements of the nervus collector. Variations occur in this arrangement as will be seen below. In this paper the last nerve taking part in the formation of the nervus collector has always been regarded as the " first girdle-piercing nerve," though occasionally, as will appear below, the foramen through which it passes is no longer complete, but is reduced to a groove. The sub- joined table shows the varying position of this nerve in the two sexes.

Table II.

Serial number

of first girdle-piercing

$

?. nerve.

Average for ? 37 '25

$ 36-43

ojr

1 Q

A

OO

36

id

37

^

23

Diff 0-82

37

26

31

( $ being more rostral.)

38

18

31

39

9

40

2

Since the serial number of the first girdle-piercing occasionally differs on the two sides of the same individual, each side has been given separately as an individual case. Consequently the 100 cases for each sex in the above table are taken from fifty specimens respec- tively. The chief point of interest here is that the males show a marked tendency to a more rostral position of the girdle, as compared with the females. This amounts to 37 -25 - 36-43 = 0'82, or nearly a whole metamere.