554 



THE MICROSCOPE, 



portion of the shell be steeped in hydrochloric acid, a 

 distinct animal structure or basis is left behind, and the 

 characters of the part will be very accurately preserved. 

 The calcareous matter, like that of bone, generally pre- 

 sents a more or less granular appearance, as at No. 4, and 

 so angular in figure as to resemble certain forms of rhom- 

 boidal crystals : No. 2 is a section from the outer brown 

 shell of the Oyster. The beauty of all such structures is 

 much increased if viewed with polarised light on the 

 selenite stage.^ 



Crustacea. — The skeletons of Crustacea are external to 

 the soft parts ; in a great number of species the shell is 

 thin and membranous, in others it is of a horny material, 

 thickened with calcareous matter, having a distinct series 



of pigment cells of a stellate 

 figure, all supplying beautiful 

 objects for microscopic examina- 

 tion and polarised light. The 

 Astacus, Crayfish, may be taken 

 as the type of that large and 

 important group of Crustacea to 

 which the term Poclophthalma, 

 Stalk-eyed, is applied." 



Cirrliopoda or Cirripeclia, 

 when mature, attach themselves 

 to rocks and other objects. The 

 Barnacle (fig. 254) and Acorn- 

 shell are tlie best known exam- 

 ples of this order ; they gene- 

 rally select floating objects to 



1, Young fry of the Oyster, a dwell upon ; and bottoms of 



{:;?;Sed ' ^^"^c. ^ ships have been covered by them 



of Paruacles. tO SUcll au CXtcut aS CVCU tO 



impede their progress through the water. The soft bodies 

 of these animals are enclosed in a case composed of five 

 calcareous plates ; from this circumstance they were 

 grouped mth the multlvalve sJiells of the older concholo- 



(1) See Prof. Huxley's article on the- "Tegumentary Organs," Cyclop. Anat. 

 o.nd I'liysio. vol. v. p. 487. 



(2) Some valuaT)le information will be found on the minute structure of shells 

 in Prof. Williams(m's paiier, " On some Histological Features in the Shells of 

 the Crustacea," Journ. Micros. Scien. vol. viii. p. 35, 18(30. 



Fig. 254. 



