1058 Marvels of the Universe 
earth. They have numerous representatives 
alive in the waters of the Mediterranean at 
the present time. 
Another type of limestone is called 
Oolitic because of the occurrence in its 
structure of numerous round and egg- 
shaped particles resembling cod-fish roe, 
3. which give it the alternative name of 
Roe-stone. 
The concentric structure of these small 
ve pin-heads of crystalline calcite is often 
arranged round a grain of sand or a 
shell fragment, but it is likely to be of 
organic origin like the pea-grits of the 
LeYNNID) CARVING lower Oolites. The remains of coral polyps 
This Crab, a native of the West Indies, spends the greater part Sante . 2 : 
ih fio cace on (i Geiine dhe cen cally diease camming Gee tho | SOMSuNEAES form the basis of limestone 
purpose of laying its eggs rocks, and although consolidated, they 
still show in many cases the shape of the skeleton built up of carbonate of lime, and the septa 
is usually filled in with crystalline calcite. 
Photo by) 
Only a few of the most interesting examples of rock-structure under the microscope have been 
here touched upon, but even these show how marvellous are some of the ways in which Nature has 
built, in her wonderful laboratory, the rocks which we so frequently regard with only the very 
slightest interest. 
LAND CRABS 
To anyone who has observed the habits of the common Shore Crab which spends most of its adult 
life above tide-mark on our shores, there should be nothing positively surprising in the statement 
that there are crabs which live habitually out of the water, though they may not have considered 
what serious modification of structure is implied in this departure from the usual habits of the race. 
There are several species, however, that have so far marked their renunciation of a marine life that 
they have ceased to develop the gills that 
are proper to that habitat, and have con- 
verted the gill chamber into a kind of 
lung, enabling them to breathe air instead 
of water. 
The best-known of these is the Land 
Crab of the West Indies, which visits the 
sea only once a year—in the spring— 
for the purpose of laying its eggs. They 
march down in myriads, allowing no 
obstacles to impede their direct course. 
The rest of the year they spend inland, even 
ascending the mountains. They burrow 
in the ground and remain in their holes 
Lin NUR during the day ; but by night come out 
N fist foe cf dio ears ato coms Tes Gels an WO Head, winen Uney millot great clmnage 
only active at night, retiring into holes for the day, or bury- on sugar plantations and other cultivated 
ing themselves in the soil. They are very destructive to 
plantations. ground. The noise made by the rattling 
LE. Step, F.L.S. 
