348 THE LIVING ANIMALS OF THE WORLD 



the arms, or " cirrhi," of many sea-worms. The superadded valves of the lamp-shells differ 

 fundamentally from those of the bivalve molluscs in the circumstance that they are developed 

 upon the upper and lower surfaces respectively of the enclosed animal, and not on the sides. 

 The union between the two valves is also accomplished through the medium of interlocking 

 calcareous teeth, in place of a horny or ligameiitous hinge-joint, as obtains in an oyster or 

 a mussel. A supplementary calcareous support, having a corresponding spiral shape, is also 

 developed in connection with the convoluted arms. This structure varies in the contour of 

 its minuter details in every specific form, which thus furnishes zoologists with a basis for 

 systematic classification. The lamp-shells collectively form two natural groups or orders. In 

 one of these the shells are hinged together, and are of conspicuously unequal dimensions. 

 The larger shell of the two is more distinctly concave and produced into a perforated beak. 

 It is this structure that in some species resembles the wick-hole of an antique lamp, and has 

 given rise to the popular title by which these shells are distinguished. The second or 

 hingelcss group is further distinguished by the shells being of almost uniform shape and size. 

 In the most interesting example of this group, known as LlNGULA, the two shells are thin, 

 horny, of a green tint, and mounted on a long, flexible, worm-like stalk. Like a worm, this 

 creature moves about in the mud, and constructs a sand-lined dwelling-tube. 



Although lamp-shells are represented by comparatively few species at the present day, in 

 the older epochs they existed in enormous abundance. It is further remarkable of this group 

 that many species are scarcely distinguishable from their fossil ancestors. Lingula, the type 

 last referred to, is especially notable in this respect. 



CHAPTER III 



STJR-FISHES, SEJ-URCHLVS, ETC. 



THE somewhat varied assemblage of marine animals familiarly known as STAR-FISH KS, 

 FEATHER-STARS, BRITTLE-STARS, SEA-URCHINS, and SEA-CTI i MIU.RS ,,11 agree 

 structurally with one another and differ from all other living organisms in several con- 

 spicuous features. Prominent among these is the circumstance that their protecting skin is 

 more or less extensively impregnated externally and strengthened internally with calcareous ele- 

 ments which take the form of plates and spines 

 and spicules. 



The COMMON SIA-IKI IIIN may be cited as 

 an example in which these calcareous elements 

 attain their maximum development. The sub- 

 spherical box-like case or shell, wherein all the 

 vital organs are enclosed and safely protected, is 

 a common object of tin- seashore, and. empty and 

 denuded of its external coating of prickly spines, 

 familiarly known as a " sea-egg." Examined closely, 

 this shell is found to consist of a series of cal- 

 careous plates, which dovetail or tit together in 

 juxtaposition with the utmost nicety. The surface 

 of the shell is studded throughout with rounded 

 hemispherical knobs, those of a larger si/e having 

 a very distinctly symmetrical plan of distribution. 

 These rounded knobs are the liases of attachment 

 of the' spines, which radiate at all points from 

 the -in face of the shell when the animal is 

 alive. It will be further recognised on a nearer 

 iftein i, tuJ /a, load in ital, **d<ak,r Cc*ri*eiiiat ,***< examination that the walls of the shell are pierced 



COMMON OR SHORT-SPINED 

 SEA-URCHIN 



