25 
always present in more or less disguised form. In all cases the adult 
or imago would appear to arise upon it just as is so obviously the 
case in the examples previously cited. In the complete paper the 
modifications of the process throughout the Metazoa would be con- 
sidered: in this place generalities alone can be dealt with. 
If the larva be laded with food-yolk, it becomes transformed into 
a more or less obvious blastoderm, upon which the imago or mature 
form takes its origin. 
Certain of the larval organs, such as those of locomotion, may 
disappear, but others, such as the larval excretory and nervous me- 
chanisms (e. g. Hirudinea according to Beran’s researches, Ich- 
thyopsida from my own work) will persist. 
Considerations of space do not permit me to enter fully into de- 
tails regarding Molluscan development. The published work on this 
group furnishes one with useful material in support of my case and 
the group is an interesting one in connection with this question of 
the relation of the larva to a blastoderm. In the Mollusca one can 
readily find all gradations from cases in which the adult is gradually 
substituted for a pelagic larva (Patella), through those in which 
the larva is somewhat burdened with food-yolk (Buccinum), to others 
finally in which there is a large yolk-sac and a blastoderm on which 
the adult form arises (Cephalopoda). Incidentally I may remark 
that it was the study of some Buccinum larvae (Veligers with 
large yolk-sac) three years ago, which first afforded a key to the 
solution of the problem of the relation of the larva to food-yolk. 
The Arthropoda are an important group, for larval forms widely 
prevail, especially, as is generally admitted, in the lower forms. 
The nature of the Nauplius is too big a question for discussion 
here, but Donrn’s conclusion that it represents a transformed worm- 
larva (i. e. one with Arthropod characters) would appear to represent 
the truth. Two of the laws governing developmental processes appear 
to be that larval organs may be transferred to the service of the 
adult, and (more usually) that adult organs may become larval, or 
as they may be termed adaptational larval organs. Numerous in- 
stances of both of these could be cited, and the three pairs of append- 
ages in the Nauplius furnish us with a case in point. 
In the development of Mysis there is an example of the con- 
version of the Nauplius larva, typically represented in the allied 
Euphausia, into a blastoderm upon a yolk-sac. 
I venture to attach most weight to the application of the prin- 
eiple to the Vertebrata; for it is there that my own work has 
chiefly lain, and it is undoubtedly the obstacles offered by the pheno- 
