^2 MOLLUSCA. 



CLASS CEPHALOPODA. 



Tlie Cephalopods are the most higlilj organized of mollusks. 

 "We recognize elements which belong to the higher and lower 

 forms of life ; the beak and complex eye, the tongue and ear, the 

 crop and gizzard, the partial segmentation of the yolk, the Cham- 

 eleon habit of changing color, the cartilaginous brain-case, point 

 to the vertebrates ; while the pharyngeal ganglion, the shell and 

 phragmocone, the tentacles and suckers, and the odontophore 

 have analogues in the invertebi'ates. 



Cephalopods are marine and carnivorous ; nocturnal and social ; 

 oviparous and bisexual — the females being larger and more 

 numerous than the males. The locomotive organs are attached 

 to the head, but the animal is able to swim by means of fins, and 

 also by ejecting the respiratory water from the " funnel." Un- 

 like many other mollusks, they are symmetrical ; and the shell is 

 nsually straight or coiled in a vertical plane, instead of being 

 spiral as in the unsymmetrical Gasteropods. The Nautilus and 

 Ai'gonaut alone of living species have external shells ; the rest are 

 naked. ^ 



ORDER DIBRANCHIATA. 



The members of this order have a pair of gills, and with the 

 single exception of the Argonauta have an internal shell, secreted 

 by the thick mantle, which is sometimes called the skeleton, but 

 which has nothing to do with protecting the nervous centres. 

 Tiie "cuttle-bone" of Sepia, the "pen" of the Squid {Loligo), 

 and the fossil Belemnites are examples. The unique shell of the 

 Paper Nautilus {Argonauta) is secreted by two webbed tentacles 

 of the female. They are well protected by eight or ten powerful 

 ai-ms, furnished with suckers, a parrot-like horny beak, perfect 

 organs of vision, vigorous circulation, and the power to secrete 

 an inky fluid with which to cover their retreat. The Giant-cuttle 

 is the largest of all animals not supported by a jointed skeleton. 

 Specimens of Architenthis have been found with a length of 

 forty feet, and jaws of much larger individuals have been found 

 in the stomachs of Sperm "Whales. The order first appeared in 

 the Lias, and is now the ruling group of the mollusks. 



