i So 



HARDWICKE'S SCIENCE-GOSSIP. 



warp the entire shell along (fig. 139). It is in this way, 

 in fact, that most of the true Echinodermata crawl 

 along the bottom of the sea. The reverent reader can- 

 not fail to be struck with such a beautiful piece of con- 

 struction, and a hint might here be furnished to our best 

 hydraulic engineers. That this principle has been in 

 vogue for myriads of years is evident by the similar 

 construction of the ancient sea-urchins. Thus in the 



Fig. 139. — Echinus climbing 



glass side of aquarium, and showing mode of attachment 

 of ambulacral sucking feet. 



ancient fossils were already in possession of the 

 hydraulic principle which has been of such inestimable 

 value to their race. The Ananchytes of the Chalk, 

 however, has very small tubercles, and the spines 

 formerly attached to them must have been very small 

 and bristle-like, as is now the case with those of the 

 living cake-urchin, Bryssits lyrifer, not uncommon 

 in the muddy bottoms of the Kyles of Bute, the 



Spatangus, Amphidotus, and 

 many others. This is not 

 the case with the Cidarids- 

 found fossilised in the Chalk 

 with them. The very large 

 knobs or tubercles on the tests 

 of the latter animals (which 

 are especially abundant in 

 tropical seas at the present 

 time) give support to large 

 spines, of a club-shape gene- 

 rally, and often ornamented 

 by various devices. Their 

 ball-and-socket principle of 

 jointing, however, was in 

 use in, and has been ever 

 since, the geological epoch 

 termed the Silurian, when 

 the Echini were first intro- 

 duced. In the Oolitic strata 

 we meet with some of the 

 handsomest specimens of 

 Cidarids, and what is very 

 peculiar is that, like the 

 fossil Oolitic corals, the fossil 

 Cidaridae resemble species 

 now living in tropical and 



Tig. 140.— Echinus esculenta; on left-hand side is a fragment of 

 test denuded of spines, and showing plates. 



"Fairy loaves," as they are called in the Eastern 

 counties, where they literally abound (the chalk 

 fossils known to geologists as Ananchytes ovatd), 

 you see five similar rows of perforations ; and even 

 the somewhat differently fashioned tests of the 

 earliest genera of sea-urchins (Palceechinus), dating 

 from Carboniferous if not from Silurian times, possess 

 p rforated ambulacral plates, showing that these very 



Fig. 141. — Test or shell of Cidaris coronata, showing the 

 tubercles to which the bases of spines are attached ; Oolitic 

 formation. 



subtropical seas. The "cake-urchins," of which 

 our recent British species of Spatangus is a well- 

 known example, date from the Cretaceous, or chalk 

 period, for the fossils are so common as to have 

 obtained the popular name of "hearts" in chalk 

 districts. These include both Spatangus, and a 

 genus called Micraster. In number of species, how- 

 ever, and variety of external form, these Echini 



