216 EDMUND B. WILSON, 
environment we find so great a difference of structure between 
the larva and the adult that no one, without knowing the 
development, could suspect their identity. They are of 
wholly unlike shape, have acquired independent and dis- 
similar sensory and locomotor organs, protective characters, 
and possibly even distinct nervous systems. Notwith- 
standing these very great differences, the peculiar develop- 
ment of the Nemertine within the larval body obviates 
entirely the necessity for intermediate stages with their 
attendant disadvantages, and permits what may fairly be 
called an instantaneous leap from one condition of adapta- 
tion to another. As in the case of Actinotrocha, a complete 
preparation for the adult condition is gradually made, by 
the formation of a large part of the adult structure from 
portions of the larval body wall, which are infolded into the 
perivisceral cavity, and there go on developing without 
altering the external features of the larva. There are four 
such invaginations in Prlidiwm, and they undergo much 
more profound and extensive structural changes than does 
the single invagination of <Actinotrocha. Yet it seems to 
me that they may have had a not very unlike origin. It is 
tolerably clear that the same general cause has determined 
their development, and I think the two processes of growth 
may be classed together. 
We find something similar, again, among the Kchino- 
derms, which are especially interesting as exhibiting a series 
of larval forms undergoing various degrees of metamor- 
phosis. Here, too, in some cases, when the metamorphosis 
is very decided, the first rudiment of the adult arises from 
an infolding of the larval body wall, though more commonly 
no such invagination occurs. But in either case the adult 
develops within the body of the larva in such a way as to 
preserve almost unaltered those external features of the 
latter, by means of which it comes into direct relations with 
the surrounding medium. The tendency is towards the 
avoidance or abbreviation of transition stages by maintaining 
the larval adaptation unimpaired as long as possible. In 
this case transition stages are not entirely avoided, for it 
seems impossible to quite bridge over in this way the very 
great structural differences between the larva and the adult. 
It is a significant fact that the development of the Holo- 
thurians is simpler and more direct than that of the Star- 
fish, Sea-urchins, or Ophiurans. This seems to depend on 
the fact that the adults of the former group are soft-bodied, 
with flexible walls, and not very different in shape from the 
larve. ‘Thus, a direct transformation of the larve into the 
