276 NATURAL SCIENCE. APRIL, 
stem of Antedon is the equivalent of the Javval stem of Antedon’s 
ancestors, would be at least a possible explanation, were it not for 
one fact: the stem-ossicles of the Antedon larva are not at all ofa 
simple type, they have a very peculiar specialised structure, and, 
broadly speaking, resemble the stem-ossicles of the Bourgueticrinide 
so common in the Upper Chalk. It is a mistake to call this larva 
‘*pentacrinoid”’ ; if any genus is known to us with which it may be 
compared, that genus is Thiollievicrinus, of the Jurassic rocks, many 
species of which show a gradual loss of the stem and development of 
cirri around the base of the cup. What the ancestors of Thiollieni- 
cvinus may have been we cannot say with certainty; this only seems 
clear, that the structure of the stem is specialised. A somewhat 
similar structure was developed in the Carboniferous Platycrinidae, 
from which, however, it is not likely that Thiollievicyinus was derived. ~ 
Such stems do not occur lower than the Carboniferous, and we must 
infer that the ancestors of both Platycrinide and Bourgueticrinide 
had stems of the primitive Paleozoic type, with round ossicles 
radiately ridged. From such ossicles all other types may easily 
have been derived. Now, to accord with Mr. Hurst’s explanation 
of the Antedon stem, it must be supposed, either that the larval stem 
of all crinoids has always been of this specialised type, which is 
contrary to all available evidence, or that its structure is a special 
larval development. Surely it is hardly probable that an organ 
which is only in use for a few days of the creature’s life should have 
become specialised in just the same way as the stem of the adult 
ancestors. It is certainly easier for the ordinary mind to imagine 
that, by acceleration of development (a principle ignored by Mr. 
Hurst), the structure of the adult ancestor has been pushed back to 
the larval stages of the existing Antedon. 
But the stem is by no means the only structure in the larval 
Antedon that bears on the Recapitulation Theory. ‘‘ Each transient 
stage in the development of an individual,” says Mr. Hurst, “isa 
modification of the corresponding stage of development of its 
ancestors.” Let us see how this is borne out by the development 
of the cup-plates. 
The most important peculiarity of the larva as distinguished 
from the adult Antedon is the presence of a plate in the anal interradius. 
At an early period this plate appears between two of the radials, and 
on the same level with them; subsequently it is lifted out from 
between the radials, which close in under it as it passes upwards, so 
that at last the radial circlet is quite continuous, and the anal plate 
lies above it. At about this stage the crown is separated from the 
stem; shortly afterwards, the orals disappear, and a little later, the anal 
plate is itself absorbed and vanishes. Now, on Mr. Hurst’s theory, 
each of these stages should be represented in the early development 
of the ancestors of Antedon; but this would raise far greater difficulties, 
for the oldest crinoids do not possess this anal plate at all, at all 
