RADIATES. 161 



2. Ophiura Family (Ophiuridee) . — Rays slender and very flexible; 

 viscera confined to the central disk of the Star-fish, as in Ophiura. 



3. Comatula Family {Comatulida?). — Rays narrow, often much sub- 

 divided ; a small supplementary series of arms on the back for 

 clinging ; and the animal usually attached by these dorsal arms 

 so as to have the mouth upward, unlike other Asterioids, as in the 

 genus Comatula. 



C. Crinoids. — There are two tribes : — 



1. The Crinidea or Encrinites. — Having a regular radiate structure, 

 and the arms proceeding from the margin of the disk, as in figs. 

 155 and 158. 



2. Cystidea (from the Greek for a bladder), fig. 156. Radiate 

 arrangement of the plates not distinct. Arms when present pro- 

 ceeding from the centre of the summit instead of the margin of a 

 disk ; in some only two arms ; often wanting, and replaced by 

 radiating ambulacral channels, which are sometimes fringed with 

 pinnules. 



The Crinids closely resemble a Comatula; only they have a stem, 

 instead of the short arms, for attachment. The stem consists of calca- 

 reous disks like button-moulds in form, set in a pile together, and 

 hence in the living animal it has some flexibility. Fig. 158 repre- 

 sents a modern Crinid [Pentacrinus Caput- Medusae) from the West 

 Indies. The mouth is at the centre between the arms. Fig. 155 a 

 represents the form of one of the disks of the stem in an extinct 

 Crinid. The disks in Palaeozoic species are generally round or 

 oval ; and they were the same also in many later species. Penta- 

 gonal disks commence in the Lower Silurian, and are most common 

 in the Mesozoic formations. The pentagonal forms represented 

 in fig. 158, a, b, c, d, pertain to the Pentacrinus family, the only 

 family of Crinids now known to exist. 



In ancient Crinids or Encrinites, the arms are not free down to 

 the pedicel, but there is a union of their lower part, either directly 

 or by means of intermediate plates, into a cup-shaped body or calyx 

 (as in fig. 155, and also figs. 527, 528, under the Carboniferous age). 



In fig. 159, the plates of one of these cups in the species Actinocrinus lonyi- 

 rostris H. are spread out, the bottom plates of the cup being at the centre. 

 The plates, it is seen, are in five radiating series, corresponding to the five rays 

 or arms of the Crinid, and between are intermediate pieces. The three plates 

 numbered 1 are called the basal, as the stem is articulated to the piece composed 

 of them ; 3, 3, 3 are the radial; 4, 4, supra-radial ; 5, brachial, situated at the 

 base of the arms ; 7 are intermediate plates, called inter-radial; 8, another inter- 

 mediate, the inter-supraradial. Sometimes, in other Crinids, there is another 

 series of plates, at the junction of the plates 1 and 3, called sub-radials. Finally, 



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