FOSSIL CRINOIDS. 189 
{the cilia having disappeared), now constituting a layer of 
protoplasm conforming to the outline of the Antedon. 
Meanwhile the cup of the crinoid has been forming. It 
then assumes the shape of an open bell; the mouth is 
formed, and five lobes arise from the edges of the calyx. 
Afterward five or more, usually fifteen tentacles, grow out, 
and the young Antedon appears, as in Fig. 129,0. The 
walls of the stomach then separate from the body-walls. 
The animal now begins to represent the primary stalked 
stage of the Crinoids, that which is the permanent stage in 
Rhizocrinus, Pentacrinus, and their fossil allies. After liv- 
ing attached for a while (Fig. 130), it becomes free (see right- 
hand figure) and moves about over the sea-bottom. 
Fig. 181. -A Blastoid, Pentrenites, seen from the side and from above.—After Liitken, 
There are two species of Antedon on the New England 
coast, one (A. Sarsti) inhabiting deep water in about one 
hundred fathoms, and the other (4. Eschrichtti Miller) 
shallower water (twenty-five fathoms) in the Gulf of Maine. 
Order 2. Blastoidea.—No forms have been discovered 
later than the Carboniferous period. The group began 
its existence as species of Pentremites (Fig. 131) in the 
Upper Silurian, and culminated in the Carboniferous age. 
It connects the Crinoids with the Cystideans ; the species 
have no arms, are supported on a short, jointed stalk, and 
the oral plates, when closed, as they are in a fossil state, 
make the calyx look like a flower-bud. There is a mouth 
and eccentric anal outlet and five radiating grooves, along 
