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, C. 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 

 Rliizocrinus, 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. 131. -A Blastoid, Penlre mites, seen from the side and from above. After Ltitken. 



There are two species of Antedon on the New England 

 coast, one (A. Sarsii] inhabiting deep water in about one 

 hundred fathoms, and the other (A. Esclirichtii Miiller) 

 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 



