394 MOLLUSC A. 



shell gland) as flattened projections beside each other (fig. 395). Later 

 the embryonic body becomes distinct from the yolk, which, enclosed in a 

 cellular envelope, remains attached to the rest, near the mouth, until its 

 substance is absorbed in the growth of the young and the animal is ready 

 for hatching (fig. 396). 



The Cephalopoda are exclusively marine. Some inhabit rocky shores, 

 others the high seas. All are carnivorous and in turn are preyed upon by 

 fishes, etc. Classification is based upon the number of gills and number 

 and character of the arms. 



Order I. Tetrabranchia. 



With four gills, four auricles, and four nephridia; numerous tentacles 

 without suckers, a well-developed chambered shell, siphon of two separate 

 parapodia, and simple eyes (fig. 384). Only four living species known, all 

 belonging to the genus Nautilus. The animals, which live in the Malay- 

 sian regions, are rare, but their shells are abundantly cast up by the sea. 

 In past time the tetrabranchs were very abundant. The NAUTILID^E, 

 with straight (Orthoceras) or coiled shells (Goniatites, etc.) flourished in 

 paleozoic times. They had simple septa. The AMMONITID^: with folded 

 septa were largely mesozoic. Since no living forms exist, their pertinence 

 to the tetrabranchiates is assumed from the character of the shell. 



Order II. Dibranchia. 



With two nephridia, two gills, and two auricles ; eight or ten arms 

 with suckers ; highly organized eyes ; shell rudimentary or absent. 



FIG. 397. Octopus bairdii.* (From Verrill.) A hectocotylised arm on the right side. 



Sub Order I. DECAPOD A. Ten arms, with lateral fins to the body. 

 Shell usually present. SPIRULID^E, with internal chambered loose-coiled 

 shell. Spirula (fig. 387). OIGOPSIDA, pelagic, with perforated cornea (p. 385) 



