HARDWICKE'S SCIENCE-GOSSIP. 



181 



are most abundant in Tertiary strata. It is a peculiar 

 law in the history of a race of organic beings— that 

 they have a period of introduction, one when they 

 reach their maximum, both numerically and in 

 variety of species, and another when these drop 

 off one by one, and the race becomes extinct. We 

 then find that the functions they performed are 

 taken up by some other kindred group of animals, 

 which, as a rule, are more highly endowed 

 and specialised, and so have been able to thrust 

 aside and extinguish their older comrades ; just as 

 British weeds are now supplanting the native weeds 

 of New Zealand and elsewhere. 



— ~\ ' 



well as in the fossil Gdarids, the mouth is at the base 

 and the anal orifice at\he summit. 



The modern Bryssuk (as we have already noted) 

 buries itself in very fine\nud, on the organic matter 

 of which it appears to fee^, just as earthworms do on 

 the black soils. The Micr»sters and Spatangi of the 

 Cretaceous period, which approach the Bryssus very 

 nearly, both as to shape ani structure, undoubtedly 

 buried themselves in the chalky \nud of the ancient sea 

 in a similar manner. Some of ths modern Echini, on 

 the other hand, appear to have thepower of hollowing 

 for themselves holes in the rocks by the sea, especially 

 in limestone rocks, which are not uiifrequently found 



Fig. j^2.—Cidaris coronata, showing" mode of attachment 

 of the club-shaped spines. 



Fig. 143. — Ananchytes ovata, or " Fairy loaf" — a common 

 Cretaceous echinoderm ; a, base, showing position of mouth 

 and anus. 



The nervous system in modern sea-urchins is placed 

 round the mouth, which is furnished with five hard 

 calcareous teeth, to enable it to triturate its food. 

 These teeth are worked by muscles, through loops, 

 and the whole can be removed as easily as an 

 artificial set of teeth. In this state the mechanism 

 goes by the name of " Aristotle's Lantern," and the 

 seaside picker-up of "unconsidered trifles" frequently 

 finds it lying by itself after the more fragile test has 

 been broken to pieces. We have seen silicified 

 specimens of Echini in Chalk flints near Norwich, 

 which have had these teeth fossilised, but such 

 examples are exceedingly rare. Nevertheless it affords 

 another instance of the persistency of a plan. Gene- 

 rally speaking, the larger number of the Echini of 

 the Chalk seas had the mouth and anal aperture at 

 the base, and such genera as Ananchytes, Holaster, 

 Micraster, Galerites, etc., are grouped according to 

 the position of these apertures, which is always con- 

 stant in the same species. In the recent Echinus, as 



Fig. 144. — Natural flint cast of interior of Ananchytes, showing 

 the perforations (in relief) for ambulacral or sucking-feet. 



Fig. 143. — Micraster, a common 

 Cretaceous echinoderm, show- 

 ing the petaloid arrangement 

 of the ambulacral areas. 



Fig. 146. — Galerites allo~ 

 galerus, a common Cre- 

 taceous echinoderm. 



riddled by them, just as they are by Pholas and other 

 boring molluscs. A pretty little sea-urchin, not quite 

 so big as a threepenny piece, which we find not 

 uncommonly fossilised in the Red Crag beds, is the 

 Echinocyamus. In some respects it is a connecting link 

 between the Echini, or sea-urchins, and the " Heart- 

 urchins," or Spatangi. The common " Sea-egg " (as 

 fishermen call it), or Echinus sphcera, is as old as 

 the Pliocene period, for we have found it in the Coral- 

 line Crag beds. The common " Sea-egg" however, 

 is not the type with which we ought to compare the 

 very abundant "Fairy-loaves" (Ananchytes) found in 

 the Chalk, and which are so plentiful about Norwich. 

 The mouth and anus of Ananchytes are both at the 

 base, whereas in the sea-egg they are relatively at the 



