MOLLUSCA 



253 



into the water, which is thus rendered opaque, and the animal 

 has a chance to escape. The sexes are separate and there is 

 no metamorphosis in development, so no larva is produced. 



In some species a curious modification of one of the anus oi 

 the male occurs in connection with reproduction. This arm is 

 inclosed in a swollen sac, which bursts and sets the arm free ; 

 this arm is called the hectocotylized arm (Fig. 262). Then in 

 some way, not known, a sac filled with spermatozoa, called the 

 spermatophore, becomes connected with this arm, which breaks 



Fie. 262. Argonanto argo, male. I, with hectocotylized arm in its sac; 2, protruded from 

 this arm contains the spermatophores or masses of spermatozoa; it eventually sepa- 

 rates from the male, passes into the mantle cavity of the female, and there the eggs are 

 fertilized. (After Miiller.) 



away from the body of the male and enters the mantle cavity of 

 the female. When first discovered there it was thought to be 

 a parasitic worm and was called Hectocotylus. There are 

 large numbers of fossil Cephalopoda; the living species may 

 be grouped in two subclasses dependent on the number of gills 

 present. 



SUBCLASS I. TETRABRANCHIATA 



The Tetrabranchiata (Gr. rerpa-, four, and fipdyxia, gills) 

 include a large number of fossil genera, but have only a single 



