428 THE MICROSCOPE. 



at present in the preparation of Indian -ink and sepia. 

 They have the power also of causing a constantly changing 

 play of various colours on their surface ; this depends on 

 the structure of the skin, which contains elastic cavities 

 filled with variously coloured fluids, acting under the 

 influence of a highly developed nervous system. Cartilage 

 enters into the formation of the head. 



Structure of SheUs. We may exhibit the structure of 

 shells by using an acid solvent in the following manner. 

 If a sufficient quantity of hydrochloric acid, considerably 

 diluted with water (say one part acid to twenty-four of 

 water), be poured upon a shell contained in a glass vessel, 

 it will soon exhibit a soft floating substance, consisting of 

 innumerable membranes, which retain the figure of the 

 shell, and afford a beautiful and popular object for the 

 microscope. In analysing shells of a finer texture than 

 such as are generally submitted to the test of experiment, 

 the greatest circumspection is necessary. So much so, 

 that M. Herissant, whose attention was particularly devoted 

 to the subject, after placing a porcelain shell in spirits of 

 wine, added, from day to day, for the space of two months, 

 a single drop of spirits of nitre, lest the air, generated or 

 let loose by the action of the hydrochloric acid on the 

 earthy substance, should tear the net-work of the fine 

 membranaceous structure. This gradual operation was 

 attended with complete success, and a delicate and beauti- 

 fully reticulated film, resembling a spider's web in texture, 

 rewarded the patience of the operator; the organisation 

 of which film, from its extreme fineness, he was not, how- 

 ever, able to delineate. In shells of peculiar delicacy, 

 even five or six months are sometimes necessary for their 

 complete development ; but in others of a coarser texture 

 the process is soon completed. Sections of shells are usually 

 mounted in Canada balsam, or in shallow cells without fluid. 



Crustacea. The skeletons of Crustacea l are external to 

 the soft parts ; in a great number of species it 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 examination. 



(1) Crustacea, from crusta* a shell. 



