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, hut 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. 1 Cuttle-fish bone, cut in thin sections, or broken 

 into small fragments, are interesting 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 thi 



(1) In the Cephalopoda -we have the first' indication of a true internal 

 skeleton, in the form of a broad flattened cartilage which protects the central 

 (optic) ganglia of the nervous system. 



