266 ELEMENTS OF COMPARATIVE ZOOLOGY. 



of chambers, only the outer one being occupied by the ani- 

 mal. Similar chambered shells are very abundant among 

 fossils. 



The mouth is armed with a pair of horny jaws shaped 

 much like those of a parrot. These are very efficient in 

 biting food; but any morsels taken into the mouth are sub- 

 jected to further subdivision by means of the lingual rib- 

 bon, which is, as its name implies, a ribbon-like membrane, 

 bearing on its surface numbers of minute teeth, which rasp 

 the food into fine shreds. 



The heart is situated in a pericardium and is systemic; 

 that is, it pumps the blood returning from the gills to the 

 various parts of the body. A peculiarity of the circulatory 

 system is that in all, except the pearly nautilus, the vessel 

 carrying blood to the gills develops a special pumping 

 organ, the branchial heart. 



The various ganglia of the nervous system are (except 

 the stellate ganglia) placed close together in the head, and 

 from this centre nerves radiate to all parts of the body, 

 those going to the tentacles being connected with each 

 other by a circular cord. 



The Cephalopods are all marine. They are carnivorous, 

 feeding upon fishes, etc. , which they capture with their 

 arms and hold fast by their numerous suckers. The 

 larger forms might be no mean antagonist for man; but 

 the monster described by Victor Hugo is without counter- 

 part in nature. The Cephalopods are divided into two 

 orders, according to the number of gills. 



OKDER L- TETRABRAKCHIATA. 



In the Tetrabranchs there are two pairs of gills (i.e., four 

 in all) ; the head bears numerous short tentacles without 

 suckers, and the body is enclosed in a chambered shell. 



