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PHYLUM ECHINODERMA 



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. 157). 



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 Ophiothrix, 

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

 Gorgonocephalus the arms are repeatedly branched. In Astronyx 

 loveni, often caught in the trawl off the north coast of Britain, the disc 

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

 Lysophiurae there is an ambulacral groove. 



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



Echinus esculentiis 



Echinoderms zvith 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, flat, 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. 



Description of the Common Sea- Urchin 



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 

 " acorn-shells " and other small sedentary animals, some 

 seaweeds, and 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 

 by five " ocular " and five " genital " plates. In the adult 



