ECHINODERMATA. 213 



tory system. The number and position of the larger orifices and the pectinated 

 rhombs constitute the principal basis for family classification. The Blastoidea have 

 orifices at the summit of the calyx which are important in classification. Some have 

 fissures at the summit, others have slits along each side of the ambulacra, and others 

 have five pairs surrounding an oral center. These openings connect with hydro- 

 spires situated beneath the ambulacra. These orifices are of family importance, and 

 some have regarded the number of hydrospires as of generic importance. 



In the nomenclature of the Blastoidea the calyx consists of the basals, radials 

 or forked plates, and orals or deltoid plates. The suture between the basals and 

 radials is the basi-radial suture. The ridge at the median line of an oral is an oral 

 or interradial ridge. In the forked plates the lower part is the body of the radial, 

 and the two prongs are the limbs. Between the limbs is the radial sinus, which is 

 occupied by the ambulacrum, consisting of a lancet-piece, which is excavated length- 

 wise by the food-groove or ambulacrum, and against it rest side plates or pore pieces, 

 marked by pinnule pits or sockets, and there are also side plates. Beneath the 

 ambulacra there are interradial systems of lamellar tubes or hydrospires. The open- 

 ings of these tubes on the ventral surface of the calyx, as in Codaster, are called 

 hydrospire slits; if they are concentrated beneath the ambulacra, as in Codonites, 

 the gap between the edge of the lancet-plate and the sides of the radial sinus is the 

 hydrospire cleft, which leads downward into the hydrospire canal. The canals open 

 externally by spiracles, sometimes called ovarian openings. The spiracles of the 

 anal interradius may be confluent with the anal opening to form the anal spiracle. 

 The plates covering the mouth and peristome, and which are sometimes continued 

 down the ambulacra covering the food-grooves, are the summit plates or the vault. 



The Cyclocystoidea have tubes radiating from the center of the disk, which 

 connect with a circular tube in the rim. It is evident there was both a circular 

 and radiate system of circulation in this order of animals. The Myelodactyloidea 

 also had a compound internal system of both circular and radiate circulation. The 

 Lichenocrinoidea attached by a base that appears to have been a single solid plate. 

 Internally there are numerous thin, upright septa radiating from the center, which 

 supported the very small external plates, and the sarcode between which must have 

 been connected with the tube in the column to have given support to it, and to 

 have maintained it in an upright position. The column tapered to a point, and no 

 evidence has been found of any external opening of these animals. The affluent 

 and effluent openings that abound in all other Echinoderms, and even among the 

 sponges, have thus far never been discovered in the Lichenocrinoidea. The notice 

 of this order in Wachsmuth's Palseocrinoidea seems to be wholly erroneous. The 

 three orders — Cyclocystoidea, Myelodactyloidea and Lichenocrinoidea — are unknown 

 in rocks later than the Upper Silurian. 



The Class Stellerida is composed of animals with a flattened and more or less 

 pentagonal body and central disk. The mouth opens in the center of the lower sur- 

 face of the disk; the skin is coriaceous, the whole body more or less flexible, and 

 along the lower surface of each arm or prolonged ray from the central disk, there 

 is a more or less distinct furrow from which the ambulacra are protruded. The Pal- 

 aeozoic orders, Asteroidea and Ophiuroidea, are exceedingly abundant in all existing 

 seas. In the common starfish the arms are mere prolongations of the disk, and the 

 plates from which the ambulacra are exserted are in deep furrows along the lower 



15 



