Class =CEPHALOPHORA=--Head-bearing. Usually enclosed in a univalve shell. Section =PTEROPODA=--Wing-footed molluscs. Section =GASTEROPODA=--Stomach-footed molluscs. Order =Nucleobranchiata=--Viscera form a nucleus on the back. Order =Opisthobranchiata=--Shell generally absent. Gills more or less exposed. Section NUDIBRANCHIATA--Naked gills. Section TECTIBRANCHIATA--Gills covered by shell or mantle. Order =Pulmonifera=--Lung-breathers. Terrestrial. Order =Prosobranchiata=. Section HOLOSTOMATA--Aperture of shell entire (sea snails). Families--_Chitonidæ_, _Dentaliadæ_, _Patellidæ_, _Calyptræidæ_, _Fissurellidæ_, _Haliotidæ_, _Turbinidæ_, _Littorinidæ_, _Turritellidæ_, _Cerithiadæ_, _Pyramidellidæ_, _Naticidæ_, &c. Section SIPHONOSTOMATA--Possess a true siphon. Carnivorous. Families--_Cypræidæ_, _Conidæ_, _Buccinidæ_, _Muricidæ_, &c. Class =CEPHALOPODA=--Sucker-bearing arms round the mouth. Order =Dibranchiata=--Two gills. Section OCTOPODA--Eight arms. Families--_Argonautidæ_, _Octopodidæ_. Section DECAPODA. Families--_Teuthidæ_ (Calamaries, Squids), _Sepiadæ_, &c. Order =Tetrabranchiata=--Four gills (containing _Nautilidæ_). CHAPTER XIII _MARINE ARTHROPODS_ The sub-kingdom _Arthropoda_ contains a vast assemblage of animals, all of which, as the name implies, possess jointed appendages. Their bodies are covered with a skin that is hardened by a horny substance (_chitin_), and frequently, also, by the deposit of carbonate of lime. The body of Arthropods is made up of a chain of segments, all of which are built up on one common pattern, and each one is surrounded by a ring of the hardened skin or exo-skeleton that gives attachment to a pair of appendages. Commonly, however, two or more of the segments become fused together, being covered by a continuous plate or shield, in which the boundaries of the rings are almost or completely obliterated; but in such cases the appendages they bear always remain distinct, so that the true number of segments is always apparent. The skin between those segments that are not so fused together remains soft and flexible, thus allowing the body to be freely bent. The appendages exhibit a great variety of structure, and are as varied in their functions. Some are used as feelers, and others as jaws for seizing or masticating food. Some are developed into powerful seizing organs for purposes of defence or attack, some into paddles for swimming, while others are legs adapted for walking.