550 THE MICROSCOPE. 



tain gases or fluids are drawn into the interior, and thus aid 

 in the supply of nourishment for the growth of the animal. 

 The cell- wall appears to bear the same relation to the 

 future perfect animal that the egg-shell of the chick does 

 to it ; it is, in fact, but an external covering to a certain 

 amount of gaseous and fluid matter, used for placing the 

 germ of life in a more favourable state for development, 

 assisted, as it is, by an increase of temperature, usually 

 the resultant of a chemical action, set up or once begun 

 in an organism and a medium. " The ovum destined to 

 become a new creature originates from a cell, enclosing 

 gemmules, from which its tissues are formed, and nutriment 

 is assimilated, and which eventually enables the animal to 

 successively renew its organs, through a series of meta- 

 morphoses that give it permanent conditions, not only 

 different, but even directly contrary to those which it had 

 primitively." 



Cephalopoda. — Molluscous animals without a foot, or 

 a distinct head, and covered with fleshy arms, bearing 

 sucker-like discs. The Cuttles and Squids form the prin- 

 cipal groups of this class, only a few species of which are. 

 found on our shores. These molluscs are the nearest 

 approach of all invertebrate animals to the vertebrate 

 forms ; and their organs of sense appear to be highly de- 

 veloped.^ Cuttle-fish bone, cut in thin sections, or broken 

 into small fragments, are intei'esting microscopic objects : 

 the peculiarities of structure are best seen when small 

 pieces are detached with a sharp knife. In the living 

 state these creatures have the power of suddenly changing 

 the colour of their skins. 



Structure of Shell. — We may exhibit the structure of 

 shell 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 



(1) In the Cephalopoda we have the first indication of a tmo intenial 

 Rkelf^ton, in the form of a broad flattened cartilage which protects tlio central 

 (optic) ganglia of the nervous system. 



