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.