134 STRUCTURAL AND SYSTEMATIC ZOOLOGY 



as Limncea and Planorbis, differ in having no eye stalks, 

 the eyes being at the base of the tentacles. They are 

 obliged to come frequently to the surface of the water 

 to breathe. 



CLASS 4. - Cephalopoda 



The cephalopods stand at the head of the branch. 

 The head is set off from the body by a slight constric- 

 tion, and furnished with a pair of large, staring eyes, a 

 mouth armed with a rasping tongue and a parrotlike 

 beak, and eight or more tentacles or arms. The body 

 is symmetrical, and wrapped in a muscular mantle. The 

 shell, if present, may be internal or external (Fig. 245). 

 The nervous system is more concentrated than in other 



invertebrates ; the cerebral 

 ganglia are partly inclosed 

 in a cartilaginous cranium. 

 All the five senses are 

 present. The class is en- 

 tirely marine (breathing by 

 plumelike gills on the sides 

 of the body), and carnivo- 

 rous. The naked species 

 are found in every sea. 

 Those with chambered shells 

 (as Nautilus, Ammonites, and 

 Orthoceras) were once very 

 abundant; more than two 

 thousand fossil species are 

 known, butonly three species 

 have been found living. 



I . Dibranchs. These are 

 the most active of mollusks, 

 FIG. 107. Cuttlefish (Sepia officinaiis}-, and the tyrants of the lower 



one fifth natural size. Atlantic coasts. , .-i ,-1 .1 



tribes. Among them are the 

 largest of invertebrate animals. They are naked, having 



