308 MANUAL OF ZOOLOGY sect. 



much simpler structure. There is a pair of otocysts, and 

 sensory processes or depressions supposed to be olfactory 

 are also present. Osphradia occur only in Nautilus. 



There are either two {Dibranchiata) or four {Tetra- 

 branchiata) nephridia, which are in the form of sacs opening 

 into the mantle-cavity, and in the Dibranchiata communi- 

 cating with the pericardium. Through each of these runs 

 one of the principal veins, round which the secreting tissue 

 of the nephridium is aggregated. 



The sexes are distinct. The ova are always large, con- 

 taining a large quantity of yolk. No metamorphosis, such 

 as is general in other groups of Mollusca, is known to occur 

 in any Cephalopod. 



The Cephalopoda are all marine, and range from tidal 

 limits to a considerable depth. Squids swim like fishes in 

 schools, rising to the surface and darting out of the water, 

 and sometimes leaping so vigorously as to fall on the decks 

 of large vessels. A large number are pelagic. They are, 

 nearly without exception, carnivorous. In length they range 

 from an inch or two to as much as fifty feet — the gigantic 

 members of the group, such as Architeuthis, being by a long 

 way the largest of invertebrate animals, and like the other 

 classes of Mollusca they are most abundant in tropical and 

 warm temperate seas. 



As already stated, the class is divided into two sub-classes, 

 the Dibranchiata and the Tetrabranchiata — the latter com- 

 prising only the Nautili (in addition to many fossil forms), 

 the former including all the rest of the living members of 

 the class. In the former the forefoot assumes the character 

 of a circlet of either eight or ten arms bearing suckers sur- 

 rounding the mouth. The funnel forms a complete tube. 

 The shell is usually internal ; when external its cavity is not 

 divided by septa. There are two ctenidia, two nephridia, 



