-A 6 



BKANCH MOLLUSCA. 



FIG. 70. 



M 



and the body, water is free to enter the mantle cavity, 

 and bathe the gills. This water may be forcibly ejected 

 through a special fold, the **'/>//<>//, the reaction pushing 

 the mollusk in the opposite direction, often with great 

 velocity. 



ORDER TETRABRANCHIATA (tet ra brang ke a'ta). 



The Pearly Nautilus (na'ti ids), of the Indian Ocean, 



is the introductory form. 

 The shell of this animal 

 is found to consist of a 

 series of chambers, ar- 

 ranged in a spiral, all 

 connecting by a siphun- 

 cle (si'fungkkl) with the 

 ultimate and larger one, 

 which contains the ani- 

 mal. These shells often 

 measure six inches in 

 diameter, and when pol- 

 ished, are very orna- 

 mental. The animal 

 has two pairs of gills 

 and numerous arms, 

 which, however, are 

 rather small and un- 

 provided with suckers. Nautili and allied forms, the 

 Ammonites (am'mon Ttz), have peopled the seas since 

 the earliest times; but, though more than two thousand 

 fossil species of this order have been described by Nat- 

 uralists, only six survive at the present time ; and these 

 all belong to the genus Nautilus, the sole living repre- 

 sentative of what was once a "mighty race." 



Nau'filus pompil'i us. Pearly Nautilus. ('.) Sec- 

 tion showing chambers and eiphuncle. JH, Mantle. 

 E, Eye. S, Siphon. T, Tentacles. C, Chambers. 

 , Siphuncle. 



