Page:Proceedings of the Royal Society of London Vol 69.djvu/25

Rh On the migration theory, we should expect to find the more rostral position of the girdle associated with a lessened number of collector- nerves. The reduction of the collector nerves, however, does not keep pace with the more rostral position of the girdle. Erom fig. 6 we see that with a change of five segments in the position of the girdle, there is associated a change of only two segments in the collector area. To explain this, we must suppose that those segments which lie just rostral to the collector area in the case where the girdle is more caudally situated, are capable of taking part in the collector when the girdle is shifted more caudally. Evidence in favour of such a view will be given later, when it is shown that in young embryos, the number of nerves forming the collector is greater than in the case of the adults (vide p. 19).

It has already been seen (Table III) that on the average, the number of post-girdle nerves is considerably greater in the male than in the female. Consequently it has been found necessary to consider the two sexes apart on this point. The way in which the number of post-girdle nerves varies with the position of the girdle is shown in the following two tables :

Table XI.

1st girdle- \ Post-girdle

piercing

nerves

7.

8.

9.

10.

A verac/e.

nerves.

6.

35

_

_

_

4

_

9-00

36

6

4

10

3

8'43

37

2

2

19

(3

2

S -13

38

12

13

4

2

7-8?

39

7

2

7-33

40

1

1

"

~

6-5

Table XII.

1st girdle-

Post-girdle

piercing

nerves

8.

9.

10.

11.

Average.

nerves.

7.

35

_

_

18

1

10-05

36

2

2

10

17

6

9-62

37

2

18

6

9-16

38

2

2

10

4

8-88