MUSEUM OF COMPARATIVE ZOOLOGY. 89 
the germ layers in the polypide bud may be subdivided into two: What 
is the significance of the outer layer of the bud? and, What is the sig- 
nificance of the inner ? 
The outer layer of the bud is derived from the calomic epithelium. 
The views of those who have studied-the formation of this inner layer 
of the cystid in Phylactolamata may be classed in two categories : 
(1) those in which it is regarded as entoderm, and the process of its 
formation as gastrulation ; and (2) those in which it is regarded as 
mesoderm. To the former class belong the views of Reinhard (’80, 
p. 208), Korotneff (’89, p. 403), and Jullien (90, p. 19) ; to the latter, 
those of Kraepelin (86, p. 601) and Braem (90, p. 116), and in this 
class the views of Barrois (’86, p. 68) and Haddon (°83, p. 543), founded 
on a priori considerations, must be placed. 
Tt seems to me that, since, as Barrois has demonstrated, there is a great 
similarity between the Phylactolæmatous and Gymnolæmatous larvæ, 
and especially since the former show evident signs of degeneration, we 
are bound to study the phenomena they exhibit in the light of our 
knowledge of the ontogeny of Gymnolæmata. 
But first it is necessary to give reasons for believing that the larva of 
Phylactolæmata is to be regarded as homologous with that of Gymnolæ- 
mata; and to do this I will first name the points of similarity in the two 
larvæ, and then try to show that the differences which exist are not suf- 
ficient to invalidate the attempt to establish a homology. And, first of 
all, it may be said that, since the adult Phylactolæmata and Gymnolæ- 
mata are strikingly similar to each other, and since no one doubts their 
close relationship, we should expect a priori that their larvæ would be 
homologous, especially since the larvæ of Qymnolæmata are admitted to 
belong to the trochosphere type, of whose ancient origin there can be 
little doubt. In the second place, the very existence of a larval stage 
in Phylactolemata is indicative of its inheritance from an earlier condi- 
tion, for two reasons: (a) because in general fresh-water life tends to 
eliminate larval stages from species which have inherited them from ma- 
rine ancestors, and tends little to form them de novo (Hydra, fresh-water 
Turbellarians, Rotifera, Oligocheta, Hirudinea, Astacus, and fresh-water 
Mollusca); and (b) because, specifically, the early stages of development 
of Phylactolæmata are passed within a uterus-like sac, from which the 
embryo is released only when a colony is already well established. In 
the third place, the Phylactolematous larva possesses, in common with 
all Gymnolematous larvæ, the following characteristics. The primary 
poly pides arise in both at a pole, and this pole is in both a prominent disk, 
