ECHINODERMA TA. 



2. Ophiurida. Skin with plates, arms simple. 



Ophiocoma, Ophiothrix, are common genera. 

 Amphiura squamata is said to be hermaphrodite. 



Class EcHiNoiDEA. Sea-Urchins. 

 Types — Echinus edulis, Strongylocentrotus lividus, etc. 



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 sea- 

 weeds, 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 a 

 flattened sphere. 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-am bulacral 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 ambulacral plates are seen to be perforated 

 by small pores, four pairs or so to each plate. Through 

 each pore a tube-foot is connected with an internal ampulla. 

 In the starfish the ambulacral areas are wholly ventral, and 

 the apical area seen on the dorsal surface of the young forms 

 is not demonstrable in the adult. 



The "posterior" ambulacra, those between which the 

 modified basal or madreporic plate lies, are often distin- 



