428 WILSON. [Vor. III. 
PART IIl. 
GENERAL QUESTIONS. 
There is reason to believe that Lumbricus is a somewhat 
specialized form, both anatomically and embryologically, and it 
is therefore necessary to be cautious in drawing general con- 
clusions from the phenomena of development, especially in 
respect to their phylogenetic significance. Yet the very fact 
of secondary modification having taken place gives value to a 
comparison of the development of Lwmdricus with that of other 
annelids, since it gives in some degree a test of the weight that 
can justly be assigned to the various features of the ontogeny. 
I may recall the well-known fact that among the annelids, as 
among many other animals, two types of development occur, 
which Balfour has conveniently designated as the /arva/ and the 
fatal types. The former, represented typically by the develop- 
ment of Polygordius, Eupomatus, etc., is indirect, and is charac- 
terized by the appearance of a free-swimming Trochosphere 
stage, in which the trunk is more or less completely suppressed 
and the head-region is highly developed. The second or fcetal 
type occurs in forms like Lumbricus, Criodrilus, Clepsine, etc., 
which undergo a direct development within an egg-capsule, sur- 
rounded by nutritive albumen. In this type the development is 
abbreviated, the free-swimming stage is more or less completely 
suppressed, and the trunk-region is early developed. [Hatschek 
has pointed out that the foetal forms may be arranged under 
two divisions, one including those like Lesmbricus, which have 
little deutoplasm in the ovum and develop by embolic invagina- 
tion ; the other comprising such forms as Evaxes or Clepsine, in 
which the ovum is heavily laden with deutoplasm and the gas- 
trulation is consequently of the epibolic type. ] 
It is furthermore generally agreed that both the larval and 
foetal forms have undergone more or less extensive secondary 
modification, the former through adaptation to the conditions of 
free-swimming larval life, the latter through simplification and 
abbreviation caused by the lack of those conditions, and perhaps 
by special adaptations caused by the peculiar mode of nutrition. 
In view of these facts it seems clear that agreement in any 
feature of development between the larval and foetal types may 
