5 5' 



ECHINODERMATA ECHINOIDEA 



ant.amb 



front 

 long 



petals, E. pennatifidum twelve to fourteen. Both come from 

 deeper water than E. cordatum. 



Fam. 7. Brissidae. Allied to the Spatangidae, but dis- 

 tinguished by sunken petals and a peripetalous fasciole. 



Two genera are recorded from the British area, Schizaster 

 and Brissopsis, but the first has only been found once in deep 



water; the second is common. 

 Schizaster has the 

 petals three times as 

 as the hind ones, and no 

 sub-anal fasciole. Brissopsis 

 has the front and hind 

 petals of about the same 

 length, and a sub - anal 

 fasciole. The only British 

 species is called B. lyrifera, 

 on account of the fiddle- 

 shaped outline of the peri- 

 petalous fasciole. 



Hemiaster (Fig. 250) in 

 general resembles Schizaster, 

 but the petals are equal in 

 length, and the two posterior 

 serve as brood -pouches for 

 the young. This genus is mainly Antarctic. 



Fossil Echinoidea. Echinoidea are well represented in the 

 geological record, and form a characteristic element in many 

 fossil faunas. They appear in the Ordovician formation, but 

 the first representatives of an existing family (Cidaridae) only 

 appear in the Permian. 



Space will only permit us to treat of the extinct members of 

 the group very briefly. Leaving out of sight the representatives 

 of families still living, the fossil Echinoidea may be divided into 

 two great groups, viz. : 



(a) Palaeozoic forms, which in some points serve to connect 

 the Endocyclica with the primitive Asteroidea. 



(&) Mesozoic forms, which serve to connect the Clypeastroidea 

 and Spatangoidea with the Endocyclica. 



The Palaeozoic forms are often called Palaeoechinoidea, and 

 they are above all distinguished by the fact that the number of 



Fig. 251. Dried shell of Schizaster, showing 

 peripetalous fasciole. ant.amb, Anterior 

 ambulacrum ; fasc, peripetalous fasciole ; 

 g.jp, genital pores. x 1. (After Agassiz.) 



