X. MOLLUSCA. 77 



by the water which they discharge with force through the funnel 

 leading from their branchial cavity. They are divided according 

 to the number of their gills, which is either two or four, into 

 Dihranchiata and Tetrahranchiata. The former breathe by a 

 single pair of internal symmetrical branchiae or gills. The eyes 

 are sessile. The mandibles are horny. They have eight or ten 

 arms furnished with rows of acetabul^e or suckers. The body is 

 sometimes laterally or posteriorly finned. The shell is either 

 internal or absent. The two-gilled section contains species with 

 eight arms, as Argonauta and Octojms, and others with ten arms, 

 as the Cuttlefishes (Sepia), the Squids (Loligo, Ommcttostrephes, &c.) 

 and Sinrula. The " shells," of the Paper Nautilus, {Argonauta), 

 are found after strong easterly gales on the outer beaches 

 of the coast at Port Stephens. The shells of Spirula are also 

 found scattered on the shores of Bondi and Coogee Bays ; a few 

 imperfect ones, containing a portion of the animal, have been 

 obtained on the coast and may be seen in spirits. A number of 

 these specimens are exhibited in a case at the North end of the 

 Upper Floor. Of the latter Section (Tetrabranchia) but one 

 representative now exists, viz., the Pearly Nautilus, breathing 

 by two pair of branchiae, having the mandibles shelly, the arms 

 very numerous and without suckers, the shell external, chambered, 

 and capable of containing the animal. 



Three species of the Paper Nautilus {Argonauta), the Pearly 

 Nautilus {Nautilus j^ompilius), and four others amongst which 

 is Sjnrula 2yeroni, will be found in Case 1. 



PTEROPODA. — The Pteiopoda are exhibited in Case I. 

 They are small molluscs, some of them even microscopic, and 

 are popularly known as " Sea-butterfiies" and " Whale food." The 

 first of these names has been given on account of their form and 

 the incessant movements of their swimming lobes; the second 

 because they form a portion of the food of the Balaena and other 

 Whales, as well as of a great number of fishes. The Pteropoda 

 live at a certain depth beneath the surface, and approach the 

 shore only when carried by storms or currents. 



Like the preceding group they are organized for swimming 

 freely in the ocean. They progress by means of a pair of fins 



