345 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 ligamentous 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 
hingeless group is further distinguished by the shells being of almost uniform shape and size. 
In the most interesting example of this group, known as LINGULA, 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 tlt 
STAR-FISHES, SEA-URCHINS, ETC. 
HE somewhat varied assemblage of marine animals familiarly known as STAR-FISHES, 
FEATHER-STARS, BRITTLE-STARS, SEA-URCHINS, and SEA-CUCUMBERS all 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 SEA-URCHIN 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 the 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 fit together in 
juxtaposition with the utmost nicety. The surface 
of the shell is studded throughout with rounded 
hemispherical knobs, those of a larger size having 
a very distinctly symmetrical plan of distribution. 
These rounded knobs are the bases of attachment 
Photo by U7. Saville-Kent, F.Z.S. of the spines, which radiate at all points from 
COMMON OR SHORT-_SPINED the surface of the shell when the animal is 
SEA-URCHIN alive. It will be further recognised on a nearer 
This species is used for food in Italy and other Continental countries @Xamination that the walls of the shell are pierced 
