192 INVERTEBRATE ANIMALS. 



funnel turned toward the keel. It swims by the jets of water 

 emitted from the funnel, and crawls upon the sea-bottom, head 

 downward, carrying its shell on its back. The male Argonaut 

 is only about an inch in length, has no shell, and has all its 

 arms alike, except the one which is metamorphosed into the 

 " hectocotylus." The Poulpes ( Octopi) are universally dis- 

 tributed in the seas of both temperate and tropical regions. 

 They are the " polypi " of Homer and Aristotle, and are vo- 

 racious animals inhabiting rocky shores. 



The Decapoda are chiefly found in the open sea, often in 

 enormous numbers, and the best known are the Calamaries 

 and Squids. The body is elongated, and is always furnished 

 with lateral fins, with which they swim actively. The shell is 

 internal, and differs considerably in different members of the 

 group. To this section of the Dibranchiata belong the sin- 

 gular fossil forms which are known to the geologist as Belem- 

 nites. These singular forms are known almost solely by their 

 complicated internal skeleton, and they appear to have abound- 

 ed in the seas of the Secondary period. 



The second order of the Cephalopoda that of the Tetra- 

 branchiata comprises forms characterized by being creeping 

 animals, protected by an external, many-chambered shell, the 

 partitions between the chambers being perforated for the pas- 

 sage of a membranous or calcareous tube, termed the " si- 

 phuncle." The arms are more than ten in number, and are 

 devoid of suckers ; the gills are four in number, two on each 

 side of the body ; the funnel does not form a complete tube ; 

 and there is no ink-bag. 



Though abundantly represented by many and varied fossil 

 forms, the only living member of the Tetrabranchiata with 

 which we are acquainted is the Pearly Nautilus, which has 

 long been known by its beautiful chambered shell. The shell 

 of the Pearly Nautilus (Fig. 91) is coiled into a spiral, and is 

 many-chambered, the chambers being walled off from one an- 

 other by curved shelly partitions or septa, perforated centrally 

 by a foramen which transmits a membranous tube or siphuncle. 

 The animal inhabits only the last and largest chamber of the 

 shell, from which it can protrude its head at will. The func- 

 tion of the chambers of the shell is not very clearly under- 

 stood ; but it appears to be that of reducing the specific grav- 

 ity of the shell to near that of the surrounding water ; since 

 they appear to be filled with some gas apparently secreted by 

 the animal. The siphuncle does not communicate in any way 

 with the chambers of the shell, and its functions are also un- 



