236 ECHINODERMA. 



The free swimming larva is a Phiteiis, very like that of 

 Echinoids. 



Ophiuroids are first found in Silurian strata. 



1. Euryalida, Skin without plates, arms simple or branched and 



capable of being rolled up. 



Astrophytoii. Gorgonocephahis. 



2. Ophiurida. Skin with plates, arms simple. 



Ophiopholis, Ophiocoma, Ophiothrix^ are common genera. 

 Amphiitra sqttaniata is hermaphrodite. 



Class EcHiNOiDEA. Sea Urchins, e.g., the common Echitms 

 edtilis, Strongylocentrotus lividus. 



Most sea urchins live off rocky coasts, and not a few 

 shelter themselves sluggishly in holes. They move by means 

 of their tube feet and spines, and seem to feed on seaweeds, 

 and on the organic matter found in mud and other deposits. 

 After the perils of youth are past, the larger forms have few 

 formidable enemies. 



Form, Skin, and Skeleton. 



The hard and prickly body is more or less spherical. 

 The food canal begins in the middle of the lower surface ; 

 it ends at the opposite pole in the middle of an apical disc 

 formed of a central plate surrounded by five " ocular " and 

 five " genital " plates. The ocular or radial plates bear 

 eye specks ; the genital or basal plates bear the apertures of 

 the genital ducts, but one of the five is modified as the 

 madreporic plate. From pole to pole run ten meridians of 

 calcareous plates which fit one another firmly ; five of these 

 (in a line with the ocular plates) are known as ambulacral 

 areas, for through their plates the locomotor tube feet are 

 extruded ; the five others (in a line with the genital plates) 

 are called inter-ambulacral areas, and bear spines, not tube 

 feet. Altogether, therefore, there are ten meridians, and 

 each meridian area has a double row of plates. On the dry 

 shell from which the spines have been scraped, the ambu- 

 lacral plates are seen to be perforated by small pores, four 

 pairs or so to each plate. Through each pair of pores a 

 tube foot is connected Avith an internal ampulla. In the 

 starfish the ambulacral areas are wholly ventral, and the 



