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Growth Changes in the Head
Growth changes in the longitudinal dimensions of the head are comparatively extensive, but tend to be masked by the smallness of the scale if they are examined in relation to the total length. Accordingly head dimensions in Table III are given as percentages of the head length measured to the 5th gill-opening. These were prepared in a similar manner to those of Table II. They show, as might be expected, a relative decrease with growth in the horizontal diameter of the eye, ranging from 3% to 9% of the head-length, but in most species between 5% to 7%. This slowing down of the growth rate of the orbital region is accompanied by accelerated growth of the postorbital region, where the increase may be mainly in the interspace between the rear edge of eye and the 1st gill-opening (Etmopterus baxteri and Scymnodon plunketi), or between the 1st and 5th gill-openings (Dalatias licha), or more or less uniformly distributed. The interspace between the rear margin of eye and the spiracle scarcely varies. In the preorbital region as a whole,
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[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]
| Species | Snout Tip to Outer Nostril | Snout Tip to Ant. Edge Eye | Horizontal Diameter of Eye | Snout Tip to Mouth | Rear Edge of Eye to 1st Gill-opening | 1st Gill-opening to 5th Gill-opening |
|---|---|---|---|---|---|---|
| J = Juveniles. | A = Large Adults. | C = Change with Growth. | ||||
| J A C | J A C | J A C | J A C | J A C | J A C | |
| Squalus acanthias | 19 19 0 | 30 30 0 | 20 14 -6 | 45 39 -6 | 34 36 +2 | 16 20 +4 |
| Squalus blainvillii | 17 18 +1 | 29 29 0 | 23 20 -3 | 47 41 -6 | 32 32 0 | 16 19 +3 |
| Etmopterus baxteri | 9.5 8.5 -1 | 30 26 -4 | 26 19 -7 | 49 40 -9 | 28 37 +9 | 16 18 +2 |
| Centrophorus squamorus | 15 16 +1 | 24 26 +2 | 30 25 -5 | 47 42 -5 | 32 33 +1 | 14 16 +2 |
| Scymnodon plunketi | 4 6 +2 | 22 19 -3 | 26 21 -5 | 38 30 -8 | 34 41 +7 | 18 19 +1 |
| Dalatias licha | 4 6 +2 | 15 15 0 | 22 13 -9 | 30 23 -7 | 43 44 +1 | 20 28 +8 |
the growth rate is much more constant than in the postorbital region, though my data do show some decrease in the preorbital length in Etmopterus baxteri and Scymnodon plunketi, and in most of the species examined a slight increase in the distance from snout tip to nostrils. The position of the mouth shows a strong correlation with the rear edge of the eye, these two levels remaining unchanged relative to each other even though the rear edge of the eye becomes proportionately further forward with growth. Some indication of the correlation is shown in Table III by the figures for the distance from snout tip to mouth, which diminish
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with growth to about the same extent as do those for the horizontal diameter of the eye. Other than this stability beween rear edge of eye and mouth (and rear edge of eye and spiracle) the most stable level in the head is the anterior margin of the eye.