ECHINOIDEA. 



231 



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. 98). 



Ophiuroids are first found in Silurian strata. 



Classification. — 



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



capable of being 

 rolled up. Astro- 

 phyton; Gorgono- 

 cephalus. 



2. Ophiurida. Skin 



with plates ; arms 



simple. 



Oph iopholis, 

 Op hiocoma, 

 Ophiothrix, are 

 common gen- 

 era. 



Amphiura squa- 

 mata is herma- 

 phrodite. 



Class ECHINOIDEA. 

 Sea - Urchins, e.g. 

 the common 

 Echinus edulis, 

 Strongylocentrotus 

 lividus. 



Fig. ioi. — Ventral surface of disc of an 

 Ophiuroid [Ophiothrix fragilis). — After 

 Gegenbaur. 



jr., Openings of genital pockets or bursae ; 7«., 

 mouth ; v. t ventral plates of arms ; sp. , spines of 

 arms ; tf. y tube-feet, at the right side these are 

 represented as retracted ; o ., the openings through 

 which they are protruded ; p. , plates around mouth 

 bearing the so-called teeth ; one of these plates is 

 perforated, and functions as the madreporite. 



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, 



