ANIMAL KINGDOM. 125 



The animal occupies the outer chamber, as in Fig. 158. These cham- 

 bered shells containing Cephalopods were once extremely numerous ; 

 but less than half a dozen living species are known, and these are of 

 the genus Nautilus. 



Modern Cephalopods are almost exclusively naked species, having 

 an internal shell, if any. In a few species, as in the genus Spirula, 

 the internal shell is chambered and coiled (the coils not touching) ; but 

 in the rest it is straight, lying in the mantle along the back, and serves 

 only to stiffen the soft body. In the Cuttle-fish it is spongy-calcareous. 

 In the Squid, or Calamary, — a more slender animal, requiring some 

 flexibility for its movements, — it is horny, and is called the pen ( p, 

 Fig. 159 p. 119). In some cases, it has a small conical cavity at the 

 lower end. In the Belemnites, a group of fossil species, it was stout, 

 cylindrical and calcareous, with a deep conical cavity, and on one side 

 the margin was prolonged into a thin blade (Figs. 792, 793). 



The mouth of the Cephalopods has often a pair of horny mandibles, 

 like the beak of a hawk in form ; and these beaks, when fossilized, 

 have been called Rhyncholites, 



2. Cephalates. — The Cephalates are divided into two groups : — 

 (1.) The Gasteropods, the group containing the Univalve shells, as 



well as some related species without shells, — the animals of which 

 crawl on a flat spreading fleshy organ called the foot (Fig. 156) ; and 

 hence the name, from the Greek, implying that they use the venter 

 (yao-Trjp in Greek), or under surface, for a foot. 



(2.) The Pteropods, which swim by means of wing-like appendages 

 (Fig. 157), — to which the name refers, meaning wing-footed (from 

 irrtpov, wing, and ttovi, foot). 



The Gasteropods, which embrace nearly all the cephalate Mollusks, have usually a 

 spiral shell, as in the common Snail, Buccinum, Turbo, etc. The mantle of the animal 

 is sometimes prolonged into a tube or siphon, to convey water to the gills; and, in this 

 case, the shell often has a canal at the beak for the passage of the siphon. The mod- 

 ern marine univalves without a beak, the Natica group and some others excepted, are 

 herbivorous, while those having a beak are as generally carnivorous. 



3. Acephals, or Headless Mollusks. — There is but one group, the 

 LameUibranchs. — These common species are well known as bivalves. 

 Between the mantle or pallium and the body of the animal lie the 

 lamellar branchiae, or gills, as is obvious in an oyster ; and hence the 

 name LameUibranchs. In a shell like Fig. 153, p. 119, the mouth of 

 the animal faces almost always (except in some species of Nucula and 

 Solemya) the margin a, or the side of the shorter slope ; and a is there- 

 fore the anterior side, b the posterior ; and, placing the animal with 

 the short slope in front, one valve is the right and the other the left. 

 The hinge is at the back of the Mollusk. 



