EABTHWOBM STUDIES. 201 



submerged on the margins of Derwentwater and elsewhere at this 

 period. Slight chemical changes produce greater or lesser de- 

 grees of hardness in the substance. Hence horny substances are 

 not all alike hard, and the horny capsule of the worm is tolerably 

 elastic ; so that when kept in a moist condition it can be slightly 

 expanded by the internal pressure exerted by the growing worm. 

 But how can the worm grow ? The chick can become no larger 

 than the shell-surrounded yolk and albumen will permit, but 

 when the young worm is hatched it will very probably be an inch 

 in length. Let us see how this contingency is provided for. The 

 following illustration will help to make the matter plain. If 

 we took a small tube of gelatine, and placed within its cavity a 

 tiny globule, we could secure the contents of the tube by drawing 

 the two ends to a point. If now the globule could expand on the 

 application of moisture, it must either burst its case, cause it to 

 expand in the direction of its shortest diameter, or force open 

 the ends of the tube. Now the egg-capsule of the worm can 

 expand slightly, but not to a sufficient extent to allow the worm 

 to reach full dimensions. Consequently the embryo gradually 

 forces open the sealed extremities of the case, and thus paves the 

 way for its ultimate escape, at the same time that it loosen its 

 swaddling bands, and develops little by little into a perfect 

 worm. While the beak of the embryo bird develops and 

 hardens within the shell sufficiently to enable it to peck its way 

 out of the calcareous covering, the worm has no such tool for 

 opening its prison-house, and so these other means must be pro- 

 vided for its escape. 



It may occur to some observant reader that a condition 

 analogous to this is found in the case of the dung-flies' eggs, 

 which are deposited with their horn-like projections upwards. 

 In both instances, if the eggs are removed from their moist 

 lodging-place, they shrivel and become lifeless. Worms again are 

 not quite alone in the possession of the power to extend the egg- 

 case during incubation. Huber long ago observed the same fact 

 in relation to the eggs of ants, and those of certain sawflies can 

 similarly expand to meet the requirements of the growing grub 

 within. 



The question now arises — How does the worm lay its eggs ? 

 Although many careful observations have been made for the 



Zool. 4th ser. vol. I., June, 1897. u 



