ECHINOIDEA. 249 



The reproductive organs lie in pairs between the arms, 

 and open into pockets or bursae formed from inturnings of 

 the skin, which communicate with the exterior by slits 

 opening at the bases of the arms. Water currents pass 

 in and out of these pockets, which probably have both 

 respiratory and excretory functions. 



The free-swimming larva is a Pluteus, very like that of 

 Echinoids (see Fig. 114). 



Ophiuroids are first found in Silurian strata. 



The Ophiuroids are usually classified according to the characters of 

 their ossicles and covering plates. Some common genera are OphiothH.v, 

 Ophioconia, Ophiopholis, Ophiitra. In the deep-water Astrophyton and 

 Gorgonocephalus the arms are repeatedly branched. In Astronvx 

 lovt'iii, often caught on the trawl off the north coast of Britain, the disc 

 is relatively large and soft and the arms very long. In the extinct 

 Lysophiurse there is an ambulacral groove. 



Class ECHINOIDEA. Sea-Urchins, e.g. the common 



Echinus ediilis. 



Echinoderms with the body covered by rows of plates, 

 usually in vertical series and forming an inflexible test ; the 

 shape of the majority approaches a sphere, but some are pin- 

 cushion-like, fiat, or obviously bilateral ; the test is covered 

 with spines which vary greatly in length and thickness in the 

 different types ; the locomotor and respiratory tube-feet usually 

 extend from the peristome to near the aboral pole ; there is 

 often a well-developed system of apical plates ; the mouth is at 

 the lower pole, the anus either at the aboral pole or in the 

 posterior inter-radius; the gonads are unpaired, five in 

 number, and inter-radial. 



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. 



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 in the young animal of a central plate surrounded 



