122 MOLLUSCA. 



and thin crystals ; these may be well examined in the 

 skin of a sprat. 



The roe of fishes consists of the ova or eggs,, and 

 the spermatozoa, the ova being contained in the 

 hard, the spermatozoa in the soft roe. The eggs 

 consist of a cell surrounded by one or two membranes; 

 and the latter are often traversed by numerous fine 

 radial canals, or present a funnel-shaped tube leading 

 to the ovum. The spermatozoa of the soft roe consist 

 of exceedingly slender filaments (fig. 25), terminated 

 at one end by a kind of head. The reader will not 

 fail to detect the analogy between the ovum of the 

 animal and that of the ovule of the plant ; and it need 

 scarcely be stated that the spermatozoa of the animal 

 fertilize the ova, in the same manner that the pollen- 

 tubes and spermatozoa of plants fertilize the ovules ex- 

 isting in them. In the case of fishes, the spermatozoa 

 of the soft roe escaping into the water, and moved by 

 the ciliary action of the filament, enter the micropyle- 

 like canals of the ova, which are deposited by the 

 fish upon the bottom of rivers. 



The scales of fishes may be prepared for examina- 

 tion by scraping them off and macerating them in water 

 until the adherent portion of the skin is softened and 

 decomposed, so that it may be washed away. They 

 should be dried between glass plates, and viewed 

 under a low power, as dry transparent objects. 



The structure of muscle can be more easily made 

 out in fishes than in other animals. A portion of 

 the flesh should be macerated in spirit as directed 

 above. 



MOLLUS'CA. We shall now leave the vertebrate 

 animals, and pass to the subkingdom Mollusca, the 

 marine kinds of which are popularly called shell- 

 fish : three of their structures form interesting ob- 

 jects for examination the shell, the tongue, and the 

 gills. 



Shell. The general structure of the shell of the 



