ECHINODERMATA. 6 



Order v. Ophiuroidea. — Body discoidal, distinct, depressed, provided with long, 

 slender arms, in which there is no excavation for any prolongation of the viscera ; they 

 are special organs of locomotion, and independent of the visceral cavity ; they have spines 

 developed from their sides ; the mouth, surrounded by membranous ten taenia, is always 

 below and central, and serves at the same time as the anus. Skeleton complicated, com- 

 posed of calcareous pieces, of which the size and number vary in different genera. The 

 arms, long and slender, are sustained internally by central vertebral-like pieces, but they 

 are not hollow or grooved underneath, as in the Asteroidea : they are special organs of 

 locomotion, independent of, and superadded to, the visceral cavity, and have numerous 

 plates or spines regularly disposed along their sides to assist in reptation. 



Type, The common Sand-star, Opldura texturata. Lamarck. 



This order is represented by one genus in the Silurian rocks. Several genera are 

 found in the Oolitic, Cretaceous, and Tertiary rocks, as well as in our present seas. 



Order vi. Blastoidea. — Body in the form of an oval calyx, composed of solid, cal- 

 careous plates, provided with five interambulacra and five ambulacra, the latter united su- 

 periorly, striated transversely, and having a deep furrow down the middle ; ten ovarial holes, 

 opening into five at the summit, and a central mouth aperture ; a short, slender stem ; but 

 the body is destitute of arms. 



Type. Pentremites injiatus. Sowerby. Carboniferous Limestone. 



The genera are all extinct, and belong to the Palaeozoic rocks. One species appertains 

 to the Upper Silurian, six to the Devonian, and twenty-four are special to the Carboniferous 

 rocks. 



Order vii. Cystoidea. — Body more or less spherical, supported on a jointed stem ; 

 the bursiform calyx is formed of close-fitting polygonal plates, varying in number in the 

 different genera, and investing the surface like a coat of mail, except above, where 

 there are three openings, one for the mouth, one for the anus, and one with a valve for 

 the reproductive organs ; the fourth aperture is below, and is continuous with the canal 

 in the stem. Some have two or four arms, others are armless ; certain species possess 

 articulated tentacula, and curious comb-like appendages, or pectinated rhombs, in connec- 

 tion with the plates. 



Type. Pseudocrinites qiiadrifasciatus. Pearce. Upper Silurian. 



This order is extinct. All the genera are found in the Silurian and Devonian rocks. 



Order VIII. Crinoidea. — Body bursiform, distinct, formed of a calyx composed of a 

 definite number of plates, provided with five solid arms, which are independent of the 

 visceral cavity, and are adapted for prehension ; a mouth and anus distinct ; no retractile 

 suckers ; ovaries at the base of the arms opening into special apertures. Skeleton compli- 



