XII. — CRUSTACEA AND LOWER INVERTEBRATA. 97 



Sub-Class III.— The OPHIUROIDEA, or Sand or Brittle 

 Stars. — The members of this sub-class are characterised by their 

 disc-shaped body, long rounded arms, and by the ambulacra! 

 grooves being covered in by a series of calcareous plates, or by soft 

 skin. There are two orders : — 



1. Ophiurid.8e, in which the arms are simple. Examples — 



Ophiopeza conjungens, Pectinura marmorata, Ophiolepis 

 annulosa, Oj^hionereis schayeri, Ophiocoma brevijyes, and 

 02yhiothrix longipeda. (See Case 2.) 



2. EuryalidSGj which have the arms branched and the 



ambulacral groove covered by soft skin. Examples — 

 Euryale asj^era and Gorgonocephalus austrcdis. (See 

 Case 2.) 



Sub- Class IV.— The ECHINOIDEA, or Sea Urchins.— In 

 this sub-class the body is enclosed in a more or less conical shell or 

 test, formed of numerous calcareous plates. These plates are 

 provided with spines, and are arranged in five ambulacral and five 

 inter-ambulacral double rows. The ambulacral plates are 

 perforated to allow of the protrusion of the tube feet, whilst the 

 inter-ambulacral plates carry the spines. The Echinoidea are 

 divided into three orders : — (See Cases 6 and 7.) 



1. Cidaridea. — The animals composing which are usually 



reo-ular, having the anal aperture in the apical region, 

 with the ambulacral areas equal and band like. Examples — 

 Cidaris metidaria, Phyllacanthus ausfralis, Goniocidaris 

 tubaria, Ce7itroste2?hanus rodgersii, Heterooentrotus mam- 

 millatus, Strongylocentrotus erythrogrammus, Salmacis 

 alexandri, Amblypneustes ovum, and Tripneustes angidosus. 



2. Clypeastridea, which have an irregular depressed 



body, the mouth being central and furnished with teeth, 

 and the anus eccentric. The ambulacral areas radiate 

 from the apex and are rosette-shaped. Examples : Fibit- 

 laria australis, Laganum pteronii, Araclmoides j^^acenta, 

 and EchinodisGiis auritus. 



