METAMORPHOSIS OF ACTINOTROCHA, 217 
adults is possible without the intervention of a very prolonged 
or distinct intermediate stage, while the rigid and regular 
coveringsof the other groups involvesa greater metamorphosis 
of the larva, which accordingly seems to require more 
gradual preparation. This can be made only during a 
period of transition or by means of some special mode of 
internal development. And the latter method, beiug more 
advantageous, has been acquired and preserved. ‘To trace 
out exactly how this process has been acquired seems im- 
practicable at present,.owing to the want of sufficiently 
minute and extended knowledge of the exact conditions of 
the larval and adult life. But it seems quite possible that 
a complete explanation of these interesting phenomena will 
in time become possible on this principle of the abbreviation 
of transition stages. 
The significance of insect metamorphoses has been so 
well elucidated, that I will add only a few words on this 
subject. In this group the structural differences between 
larva and adult are, in many cases, of such a nature that a 
transition stage cannot be avoided; and we find this stage 
provided, in many cases, with the most elaborate adaptive 
structures to compensate for the disadvantage thus necessi- 
tated. It is a very interesting fact, as Mr. Balfour well 
remarks, that the intermediate (pupa) stage as thus pro- 
tected ‘has become secondarily adapted to play a part in 
the economy of the species quite different from that to 
which it owes its origin,” 2. e. the protection of the creature 
during the inclement season of the year, as when the insect 
passes the winter in the pupa state. 
Although a transition stage occurs, it is ordinarily assumed, 
so far as external features are concerned, by a sudden change 
and not by gradual modification of the larva; and the change 
from pupa to imago is equally abrupt. Sir John Lubbock’s 
explanation of the suddenness of these changes is that it 
results from the hardness of the external skin, which renders 
gradual modification impossible. This is clear enough in 
the change from pupa to imago, but it seems insufficient to 
explain the sudden change from the soft-skinned larva to 
the pupa. This is rather to be explained as a result of the 
advantage of maintaining complete adaptation to larval life 
up to as late a period as possible. It is very interesting to 
note that in some cases of decided metamorphosis the abrupt- 
ness of change is facilitated by the development of portions 
of the adult organism from invaginations of the larval skin 
—the imaginal discs—so that an internal preparation 1s 
made by a process which has analogies with the develop- 
VOL, XXI.—NEW SER, P 
