LARVAL FORMS I THEIR NATURE^ ORIGIN AND AFFINITIES. 383 
following way : — There is a greater chance of the ancestral 
history being lost in forms which develop in the egg ; and 
masked in those which are hatched as larvae. 
The evidence from existing forms undoubtedly confirms 
the a priori considerations just urged.^ This is well shown 
by a study of the development of Echinodermata, Nemertea, 
Molluscay Crustacea, and Tunicata, The free larvae of the 
four first groups are more similar amongst themselves 
than the embryos which develop directly, and since this 
similarity cannot be supposed to be due to the larvae having 
been modified by living under precisely similar conditions, 
it must be due to their retaining common ancestral charac- 
ters. In the case of the Tunicata the free larvae retain 
much more completely than the embryos certain characters 
which are known to have been ancestral. 
Types of Larvae, — Although there is no reason to suppose 
that all larval forms are ancestral, yet it seems reasonable to 
anticipate that a certain number of the known types of larvae 
should resemble the ancestors of the more important phyla 
of the animal kingdom. 
Before examining in detail the claims of various larvae to 
such a character, it is necessary to consider somewhat more 
at length the kind of variations which are most likely to 
occur in larval forms. 
It is probable a priori that there are two kinds of larval 
forms, which may be distinguished as primary and secondary 
larval forms. Primary larval forms are more or less modi- 
fied ancestral forms, which have continued uninterruptedly 
to develop as free larvae from the time when they constituted 
the adult form of the species. Secondary larval forms are 
those which have become introduced into the ontogeny 
of species, the young of which were originally hatched with 
all the characters of the adult ; but which, owing to the loss 
of food-yolk in the egg, or some other cause, have become 
hatched at an earlier period. Such secondary larval forms 
may resemble the primary larval forms in cases where the 
ancestral characters were retained by the embryo in its develop- 
ment within the egg ; but in other instances the characters 
they have are probably entirely adaptive. 
* It has long been known that land and freshwater forms develop without 
a metamorphosis mueh more frequently than marine forms. This is pro- 
bably to be explained by the faet that there is not the same possibility of 
a land or freshwater species extending itself over a wide area by the agency 
of free larvsc, and there is, therefore, much less advantap in the existence of 
such larval ; while the fact of such larvm being more liable to be preyed upon 
than eggs, which are either concealed or carried about by the parent, might 
render it absolutely disadvantageous for a species to have sucli larva. 
