A SIEVE-LIKE MEMBRANE {N LEUCOSOLENIA. 259 
ectoderm—a conclusion which, it must be confessed, is still 
in need of further developmental facts to raise it above the 
rank of a probable hypothesis. If now we go further afield, 
and try to find something with which to compare it, we are at 
once struck by the great resemblance it presents, in many 
points, to the sieve plates of Euplectella, Holascus, and 
Hyalonema, among Hexactinellida. In fact, Schulze’s 
figure! of Euplectella suberea, Wyv. Thomson, looks at 
first sight almost as if it had been drawn from a preparation of 
my Ascetta. But of course there is an enormous difference 
between the two, not only in size, but in structure, since the 
membrane of Euplectella is mostly made up of spicules. In 
the sieve membrane of Ascetta, however, there is a thin layer 
of jelly between the two layers, and it is not very difficult to 
imagine how this layer might be invaded by scleroblasts, and 
come to contain spicules. It is evident that, if the gastral 
cavity and osculum of Ascetta were to grow to the size of 
that of Euplectella, a support of spicules would be necessary 
for the sieve membrane, and doubtless would be acquired. If 
this homology between the oscular sieve membrane and plate of 
Ascetta and Euplectella respectively be true, it would show 
that the osculum of Euplectella is a true osculum, and its 
gastral cavity a true gastral cavity,’ since it can hardly be 
1 Schulze, ‘Monograph of Hexactinellida,’ pl. v, fig. 1. 
? Apart from any considerations about the oscular sieve plate there can 
hardly be any doubt that the internal cavity of a sac-like Hexactinellid is a 
true gastral cavity, especially if one considers the young forms figured by 
Schulze on pl. liii, and also on pl. lxii, fig. 5, of his beautiful monograph. 
Schulze has further shown in the clearest manner how this simple gastral 
cavity may become modified. “By the expansion of the upper oscular 
margin many species acquire a funnel-like shape. A further widening and 
flattening leads to the formation of a flat saucer-like body, while a more 
unilateral growth results in an ear or shell-like form, . . . or even in certain 
circumstances in a simple perpendicular plate-like form.” ‘If the outer 
margin of a stalked or originally cup-shaped sponge becomes folded outwards 
and downwards through great development of the median portion, a fungoid 
form arises. . . . In this way, then, as the gastral cavity and osculum have 
thus been lost, what was originally the internal gastral has become the upper 
and outer surface, so that the water enters the body from below and escapes 
