424 PHYSIOLOGY. 



with the other transformations of the body ; and, besides, we have not 

 yet at all spoken of the great discrepancy of the form of the limbs, nor 

 even of what is still more important, namely, the addition of new ones. 

 In the explanation of these subjects which we are now entering upon, 

 the insects with a perfect metamorphosis will chiefly occupy us, in so 

 far as in them only does a true transformation take place ; whereas we 

 shall speak of the insects with an imperfect metamorphosis only where 

 we take notice of the moulting, and upon our investigations into the 

 sprouting of the wings. We shall here, therefore, have an opportunity 

 of circumstantially referring to that law laid down by Von Bar, that 

 there is visible in the development a perfecting as well by the means of 

 morphological and histological separation as by the progressive forming 

 of a particular figure from one more general. 



If an Arthrozoon, whose form consists of a longitudinally distended 

 and generally hardened case, composed of limbs and rings, is to enlarge 

 by growth, it must strip off its former covering and clothe itself with a 

 new one, as the old one interrupts the universal distension, and, indeed, 

 makes it wholly impossible. It is only in those Arthrozoa which dwell 

 in moist places, so that from their place of abode their integument 

 cannot harden in the air, which, therefore, constantly remains equally 

 soft and flexible, the casting of the external integument is rendered 

 unnecessary, and they therefore do not moult, but even in the higher 

 Annulata, for instance, in the leech, a moulting is observed, and still 

 higher, for example, in the Malacostraca, it is the necessary condition 

 of growth. In insects, also, this change of skin must likewise take 

 place so long as they grow, and it is this change of skin alone which, in 

 insects with an imperfect metamorphosis, presents itself as the external 

 mark of metamorphosis ; but it is also proper to insects with a perfect 

 metamorphosis, among which it indicates, as well as among the preceding, 

 a transition from one stage of life to another. 



The earlier physiologists differ in opinion from the moderns upon 

 the mode in which this new skin originates beneath the former. 

 Swammerdamm and Bonnet were of the opinion, in accordance with 

 the general idea of their age of the theory of encasement, that all new 

 skins already existed beneath the old one, and that the latter, without 

 any re-production upon the part of the larva, was merely stripped off. 

 Exclusively of the true object of moulting being overlooked in the 

 adoption of this opinion, the mere observation of the larva having con- 

 siderably increased in size immediately after the divestment, contradicts 



