C R IT 



[ 116] 



CRT! 



petals are placsd in the form of a 

 cross ; this is particularly the case 

 in a very large order of plants, 

 which have four petals, so arranged 

 as to resemble a cross. 



CRU'CIBLE. (crucilulum, Lat.) A 

 vessel, or melting-pot, made of 

 earth, so named, according to some, 

 from its having been formerly made 

 in the shape of a cross; but, 

 according to others, from the 

 metals being tortured in it by fire 

 to compel them to become gold. 



CRU'RA. (The plural of crus, Lat.) 

 Applied to parts from their resem- 

 blance to legs ; the legs. 



CRT/RAL (crural, Pr. cruralis, Lat.) 

 Belonging to the leg. 



CRUST, (krusle, Germ, crusta, Lat. 

 croute, Fr. crosta, It.) Any shell, 

 hard coat, or external covering. 

 That portion of our globe which is 

 accessible to our inspection and 

 observation is called by geologists, 

 the earth's crust. It is this crust 

 which offers proper occupation to 

 the geologist. The greatest depth 

 to which he has been hitherto 

 able to extend his observations, 

 from the uppermost strata to the 

 lowest beds, is from eight to ten 

 miles ; a thickness which, compared 

 with the bulk of the earth, does 

 not exceed that of the thickness of 

 the paper which covers a globe a 

 foot in diameter. The inequalities 

 and crevices in the varnish applied 

 over the surface of such a globe 

 would fairly represent, and be in 

 proportion to, the highest mount- 

 ains and deepest valleys of the 

 world. The mean density of the 

 earth's mineral crust has generally 

 been taken at 2'5 : according to 

 De la Beche 2'6 would be a nearer 

 approximation. 



" The term is not used with the 

 intention of conveying an opinion 

 that the earth consists only of a 

 crust, or that its centre is hollow ; 

 for of this we know nothing." 

 Phillips. 



Mr. W. Hopkins gives 800 miles 

 as the minimum thickness of the 

 solid external crust of the earth. 

 CRTJSTA'CEA. } (from crusta, Lat.) 

 CRUSTA'CEANS. ) The Crustacea pos- 

 sess a hard external covering, and 

 numerous articulated limbs; an- 

 tennas, and palpi; a heart, with 

 circulating vessels and gills, and a 

 nervous system. The crab, lobster, 

 sea urchin, shrimp, &c., are ex- 

 amples. Crustaceous animals pos- 

 sess the most solid form of the 

 skeleton met with in the articulated 

 classes. It is found in the larger 

 decapods to contain nearly half its 

 weight of carbonate of lime, and 

 there is also a considerable pro- 

 portion of phosphate of lime, with 

 traces of magnesia, iron, and soda. 

 These substances are exuded from 

 the surface of the true skin, along 

 with a tough coagulable animal 

 gluten, which connects all their 

 particles, and forms a thin varnish 

 on the surface. The Rev. J. 

 Williams suggests, " instead of 

 supposing these animals to secrete 

 the calcareous coverings which 

 they inhabit, say that they emit or 

 secrete a gluten, to which the 

 calcareous particles adhere, and 

 thus the shells are formed." The 

 colouring matter is generally be- 

 neath this varnish, and on the 

 exterior surface of the calcareous 

 deposit, but sometimes it pervades 

 the whole substance of the shell. 

 Dr. Rob. Grant. Professors BucTc- 

 land and Fyfe. 



The crustaceae respire by means 

 of branchiae; these branchiae, some- 

 times situated at the bottom of the 

 feet, at others on the inferior ab- 

 dominal appendages, either form 

 pyramids composed of lamina in 

 piles, or bristled with setae ; and 

 in some cases consist seemingly 

 wholly of hairs. The crustaceae 

 differ from the testaceae in one most 

 striking point of view : lobsters, 

 crabs, &c., cast their shell or cover- 



