554 



THE MICEOSCOPB, 



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. 1 



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 Podophthalma, 

 Stalk-eyed, is applied. 2 



Cirrhopoda or Oirripedia, 

 when mature, attach themselves 

 to rocks and other objects. The 

 Barnacle (fig. 254) and Acorn- 

 shell are the best known exam- 

 ples of this order; they gene- 

 Fig. 254. rally select floating objects to 



Young fiy of the Oyster, a dwell Upon ', and bottoms of 



d ships have been covered by them 

 of Barnacles. to such an extent as even to 



impede their progress through the water. The soft bodies 

 of these animals are enclosed in a case composed of five 

 calcareous plates j from this circumstance they were 

 grouped with the multivalve shells of the older, concholo- 



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

 and Physio, vol. v. p. 487. 



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

 in Prof. Williamson's paper, "On some Histological Features in the Shells of 

 the Crustacea," Journ. Micros. Scien. vol. viii. p. 35, 1860. 



