CEPHALOPODA 605 



was created by Cuvier in 1795 to include the same animals it does today. 

 The epoch-making dissection of Nautilus by Richard Owen resulted in 

 his subdivision of the class in 1832 into the Tetrdbranchiata and 

 Dibranchiata, and also the subdivision of these orders substantially as is 

 still done. 



Distribution and Habits. Cephalopods occur only in the sea, either 

 in the neighborhood of the coast or in the open ocean or the deep sea. 

 All are carnivorous, eating great numbers of fish, mollusks, and crusta- 

 ceans, and often being very destructive to the fisheries. Many kinds, as 

 the small squids, swim about in schools, while others, as most octopods, 

 live singly. All cephalopods swim, often with great swiftness, by squirt- 

 ing water out through the siphon, while the squids also swim by means 

 of the fins. The animal carries its body in a horizontal position while 

 swimming, with the posterior side and the siphon undermost. The octo- 

 pods often crawl about on the bottom on their arms. Cephalopods are the 

 largest invertebrates and among the largest existing animals. Architeuthis 

 princeps is the species containing the largest known individuals, the larg- 

 est examples of which are over fifty feet long, including the arms. 



Cephalopods are of use to man in various ways. Many are used for 

 food. The common squids are used as bait in the cod fisheries. The 

 calcareous cuttle bone of the cuttle fish (Sepia) was formerly employed 

 as a medicine, and the ink of the same animal is a common artist's mate- 

 rial. The shell of Nautilus is also much used in the arts, and for a 

 variety of useful purposes. 



The class contains two orders and about 400 species: about 5,000 

 fossil species are known. 



Key to the orders of Cephalopoda: 



Oj Shell large, coiled, chambered, external 1. TETBABBANCHIATA 



a, Shell not coiled (except Spirula), internal, often absent. .2. DIBEANCHIATA 



ORDER 1. TETRABRANCHIATA. 



Shell external and coiled and divided by partitions into chambers 

 (Fig. 952), all of which are empty and filled with air or a gas, except the 

 outer one, which contains the body of the animal, and may be as large 

 as all the rest of the shell ; septa regularly curved and concave in Nautilus, 

 but in the fossil ammonites often very complex in structure, with a mem- 

 branous tube called the siphuncle extending from the body of the animal 

 through all of them, which is central in position in Nautilus and marginal 

 in the ammonites ; shell composed of 2 layers, of which the inner is pearly ; 

 4 ctenidia, 4 kidneys, and 4 auricles, but no branchial hearts present; 

 siphon consists of 2 separate lobes (the epipodia) ; tentacles numerous 

 and without suckers; eye without lens; ink sac absent: about 4 living 



