The Sea-side Book. 2471 



naturalist much of the history of the shell of which it is the debris. The importance 

 of such a fact to the geologist is obvious, but I speak of it here chiefly as affording 

 an instance of the wonderful skill with which these humble works of an unseen Worker 

 are constructed. ' Lo, these are parts of his ways, but how little a portion is heard 

 of Him ! ' "—p. 34. 



" Much lower in the scale of being than bivalve Mollusca, but elaborately or- 

 ganized, and offering many interesting points in their history, are the heart urchins, 

 a tribe of animals enclosed in egg-like shells, coated with spines, which inhabit all 

 our sandy bays. There are several recent British species, but I shall only mention 

 the common heart urchin (Amphidotus cordatus), mermaid's head, or sea egg, as it is 

 variously called, which is found all round the coast. When alive, it is thickly clothed 

 with fine hair-like spines, each of which is articulated at base with a minute nipple, 

 forming a ball-and-socket joint, so that the spine can move freely in all directions. 

 The spines are of different forms and length on different parts of the body, and, frail 

 as they appear, serve the purpose to which they are applied, of enabling the animal 

 to sink itself in the sand, shovelling the fine particles out of the way, and throwing 

 them over its back. When thrown upon shore, the spines are usually more or less 

 broken, and soon are completely worn off, when the dead shell resembles a heart- 

 shaped egg, of a dirty-white colour, frosted over with minute tubercles, which are 

 largest on its under surface, where the orifice of the mouth is seen ; and it is marked, 

 both on the back and lower surface, with five radiating smooth depressions, bordered 

 with a double row of pin-holes. These spaces, which are much more developed on the 

 back than on the oral surface, are called ambulacra ; and through the pores or pin- 

 holes which border them, the animal protrudes long worm-like suckers, which serve 

 the office of feet, and enable him to move about by a sort of warping motion (to speak 

 nautically), fixing the sucker of one fibrous cord in advance of his position, gradually 

 bringing the rest forward, and so dragging the body along. Those on the oral sur- 

 face are much less developed, and chiefly serve to hold the ground. It is curious to 

 find a creature whose organs of locomotion are most developed on the upper surface ; 

 but we may be assured that they are not so placed without a wise design. It is easy 

 to see that such an organization enables the creature to recover its natural position 

 with ease, if accidentally inverted ; but the arrangement probably serves many other 

 purposes. 



" The affinity of the heart urchin with the common egg urchins is readily seen ; 

 their connexion with starfishes is, at first sight, less obvious. Nevertheless, a careful 

 comparison of the living animals will show many points in common : thus the five- 

 rayed ambulacra on the back of the Amphidotus represent the rays of the starfish ; 

 and when we place a large number of species, recent and fossil, under review, the pas- 

 sage from the most branching starfish to the roundest sea-egg may be clearly made 

 out through a beautiful gradation of forms. * * * The family of Echinidae, to 

 which these animals belong, was much richer in forms in the earlier world than it at 

 present appears to be ; and from the great facility with which the hard parts of the 

 shelly integument may be preserved, the remains of these creatures have come down 

 to us in a very perfect state. The study of them, therefore, is quite as interesting to 

 the geologist as to the zoologist. It is of importance to the former to know the habits 

 of the living species, that he may form a judgment on what those of the extinct kinds 

 may have been, and thus arrive at just conclusions on the circumstances under which 

 the rocks and gravels, where their remains are preserved, have been deposited. Of 



