200 PROBLEMS OF RELATIVE GROWTH 



appears to be mainly or solely a mechanical effect due to an 

 alteration in the direction rather than the amount of growth 

 of the mantle-edge, and to be determined by an increase in the 

 number of embryos contained in the gills. 



Similarly, Huntsman (1921) found that light not only 

 markedly retarded the growth of mussels (Mytilus), but also 

 affected their shape. Although the depth of the shell relative 

 to length was not altered, the relative breadth was increased 

 from 42 to 46 per cent ; as result, the ratio of weight to the 

 product of (/ X d X br) of course increased. The light 

 appeared to inhibit the growth of the edge of the mantle, which 

 is responsible for additions in length and in depth. Relative 

 breadth, on the other hand, again depends on the angle at 

 which the mantle-edge is growing. 



Entz (1927), in an exhaustive study on Peridinians, has 

 shown that the relative length of the horns is markedly altered 

 by salinity, while the size of the body is little affected. In 

 different external conditions, the antapical horn may vary in 

 the ratio of 4 to 1 ; the post-equatorial horn in the ratio of 

 5 or 6 to 1 ; and the other two horns from zero to a con- 

 siderable development. As Entz puts it, the relative size of 

 the horns renders small differences of salinity visible in an 

 obvious and exaggerated form. (See also Gajewski, p. 259.) 



What again appears to be a comparatively simple influence 

 is that revealed in what Przibram (1925) calls the ' Tail- 

 thermometer ' of rats and mice — namely, the fact that the 

 relative tail-length of these rodents (pp. 22, 40) increases 

 with increase of the external temperature in which they grow 

 up. The relationship is an approximately linear one. If rela- 

 tive tail-length be expressed as trunk length per cent, the value 

 of this index of tail-length (for young Albino rats) decreases 

 by 075 for each rise of i° C. in external temperature. He has 

 also shown that this is correlated with an actual increase of 

 internal temperature (0-2° C. for each 5 of external increase) ; 

 and further, that this is really the important agency in pro- 

 ducing the effect. We may therefore conclude that the effect is 

 due in part to a differential temperature-coefficient for tail- 

 growth. On the other hand, the fact that the tail (and other 

 organs such as ears and limbs, which also show a similar effect 

 with temperature) have a relatively large surface, and therefore 

 a lower temperature than the rest of the body (cf. Crew's and 

 Moore's proof of this fact for the mammalian scrotum), doubt- 

 less enters into the picture. Higher internal temperatures, 

 especially with higher external temperatures, would reduce the 



