SPATANGOIDEA 



553 



ing points, viz. : — (1) tlie shape and position of the peristome, 

 (2) the characters of the " petals," and (3) tlie number and 

 position of tlie fascicles. 



Peristome. — In many genera this is pentagonal and central, 

 and in tliese cases the interradii commence at the peristome with 

 a single plate, which is often covered with a thick crowd of small 

 spines and is termed a " bourrelet." The oral ends of the radii 

 also often consist of a crowded series of narrow plates, looking 

 something like a " petal," and termed a " phyllode." The five 

 bourrelets and five phyllodes constitute a flower-like figure termed 

 a " floscelle." 



Ambulacra. — In Echinoneus all five are alike and are provided 

 with similar tube-feet, which are respiratory but possess suckers. 

 The ambulacra are not grooved, and the petaloid arrangement of 

 the pores is hardly marked ; but in Cassidulus, Pourtalesia, and 

 many other genera the five petals are well marked, though they 

 are all similar to one another. 



Fascioles. — These structures are often entirely absent ; the 

 sub-anal one alone is present in Spatangus. In Uupatagus a 

 peripetalous one is added. This surrounds all the " petals," and 

 has obviously the function of sweeping fresh water over the 

 respiratory tube-feet. In Ecliinocardiv.m., as we have seen, there is 

 an " internal fasciole " between the two anterior petals which has 

 a similar function. In addition, this genus 

 possesses an anal fasciole which surrounds 

 the anus and sweeps away the faeces. 



The Classification of the Spatangoidea 

 is based mainly on the degree of develop- 

 ment of the petals, that is to say, on the 

 extent to which the burrowing habit has 

 been developed. But weight is also laid 

 on the shape of the peristome, the penta- 

 gonal form being more primitive. Seven 

 families are recognised, which are as 

 follows : — 



Fam. 1. Echinonidae. — "Petals" 

 hardly marked at all ; peristome in the 

 centre of the lower surface and pentagonal 

 developed. 



One genus, Echinoneus (Fig. 248). 



Young Echino- 

 neus to show five equal 

 radii scarcely petaloid. 

 amh, Arabulacral area ; 

 y.j), genital pore ; mad, 

 niadreporite. (After 

 Agassiz.) 



Floscelle not 



