616 ¥. H. GRAVELY. 
different larvee begin their life on the sea-bottom varies very 
greatly in different species; and that the ratio of the length 
of embryonic life in the jelly or brood-pouch to that of the 
free-swimming larva is equally variable. And it is obvious 
that this must have its effect on the production and suppres- 
sion of functional larval organs. 
Yet in spite of the tendency, under certain conditions, 
towards the suppression of larval organs, it is evident that 
wherever the pelagic habit is maintained for any length of 
time the body of the larva is encircled by powerful cilia 
arranged in single lines, of which not more than one is nor- 
mally present on each segment; and it is to these larve 
alone that I propose to refer in the following pages. 
Characteristics of apparent systematic value in larve may 
be due to either of two distinct causes, which it is necessary 
clearly to distinguish from one another on account of their 
different phylogenetic values. Hither they may be due to 
inheritance by all the members of the group whose larve they 
characterise, from a common pelagic ancestor or from the 
pelagic larva, specially modified through the agency of natural 
selection, of some common but less remote creeping ancestor; 
or they may be due to some correlation between adult and 
larval structures. This correlation may, and undoubtedly 
does, often show itself in the precocious development of the 
structures themselves; or its presence may be much less 
obvious at first sight. This may be illustrated by reference 
to the definitely modified larvee of the highly specialised worms 
forming the family Cheetopteridee. 
In the adult the prostomium is reduced to a minimum, a 
reduction which begins to show itself in the case of Che top- 
terus variopedatus Reni, (= pergamentaceus, Cuv.) in 
quite young and unsegmented larvee (see Wilson, 1882, Pl. 
XXII, figs. 82, 83) and becomes more and more marked as 
development proceeds ; thus by the precocious appearance of 
an adult character a larva is produced whose form renders the 
possession of a powerful prototroch an impossibility; and 
so the prototroch is entirely suppressed and replaced by a 
