PASSIVE DEFENCE 



335 



sharp spines are connected with some of the fins, and as it is the 

 dorsal ones which are best off in this respect, we may infer that 

 such fishes are most Ukely to be attacked from above by enemies. 



Fig. 493- — Method of growth in Gastropod Shells. The three upper iigures show upper surface, while the two 

 others are side views, a, h, c, d, e, represent successive additions to the shell. 



Cat-Fishes {ChimcBra and Callorhynchus), the Piked Dog-Fish 

 {Acanthias), and Perch-like Fishes may be taken as examples. 



Many of the Mollusca are more or less perfectly enclosed in 

 shells, which receive successive additions as their owners increase 

 in size, and these are usually indicated by 

 lines of growth visible on the outer sur- 

 face. The shape of a Gastropod varies 

 in shape according to the way in which 

 additions are made (fig. 493), and the 

 same thing is also true of the shell of a 

 bivalve. A shell of the latter kind is 

 shown in section in fig. 494, and the 

 various stages of its growth are indicated 

 in a diagrammatic way. 



Molluscan shells may be either of the 

 nature of plate-armour, or may be pro- 

 vided with sharp projecting spines. 

 Among the former are Pearly Nautilus 

 {Nautilus pompilius), Garden Snail [Helix 

 aspersd), and Freshwater Mussels {Unio and Anodonta). 



Among the spiny Sea- Snails may be mentioned species of 



Fig. 494. — Growth of Bivalve Shell as 

 seen in section, c. Oldest part; a, later 

 additions; b, youngest layer represented. 



