372 WILSON. Vor aoe 
takes place in the region of the cubical cells, and the larval 
epithelium is not lost, but is modified cell by cell. 
The larva of Amorphina (Schmidt, 22) is a solid larva ciliated 
all over. The cilia are lost at the posterior end. In the same 
paper, Schmidt describes the larva of a species of Esperia. The 
larva is solid and is ciliated all over. The cilia are lost at one 
pole, the spicules collecting at this pole. The larva of Reniera 
(Schmidt, 2. 7.) is a solid ciliated form with a deeply pigmented 
pole. The cilia on the pigmented pole are lost. There are 
other observations by Metschnikoff (11) and Carter (2) to the 
effect that the ectoderm is absent at the posterior pole of the 
larvae of silicious sponges. In these cases, as in the case of 
the Myxilla larva described by Vosmaer, it remains doubtful 
until the early development is known, whether the larva is 
really an egg-larva. 
It will be seen that the larvae of the above-mentioned 
silicious sponges agree in fundamental respects. They all 
consist of two germ layers: an inner parenchymatous mass 
(mes-entoderm) and an outer layer of columnar ciliated cells 
(ectoderm). At one pole, usually the posterior, the ectoderm 
is apparently absent, the appearance being probably due to the 
fact that at this pole it is composed of flat unciliated cells. 
Like the egg larvae, the gemmule larvae (of Esperella and 
Tedania) consist of two germ layers, an inner parenchymatous 
mass (mes-entoderm) and an outer layer of columnar ciliated 
cells (ectoderm), the columnar ciliated cells giving place to flat 
unciliated cells at the posterior pole which is thus differenti- 
ated. It is plain that the two sorts of larvae agree in essential 
structure. 
There are other silicious sponges, Spongilla (6, 14), the 
tetractinellid form Plakina monolopha (26), and Tedanione (see 
ante, p. 345), in which the larva has not the peculiar differenti- 
ation of one of the poles which is seen in the above-mentioned 
forms. But this differentiation is so common that it may fairly 
be considered as typical of a large though ill-defined group of 
sponges. 
Cause of the Resemblance.— Accepting as a fact the resem- 
blance between the egg and gemmule larvae in the possession 
