424 



HISTORICAL GEOLOGY. 



The Calamary or Squid, Loligo vulgaris (length of body, 6 to 12 

 inches) ; i the duct by which the iuk is thrown out; p the " pen." 



or 5 pairs of arms furnished with tentacles, suction-disks, or horny claws, as in the Ceph- 

 alopods. 



The subdivisions are as follows : — 



1. Cephalopods. Free-swimming ; having 4 or 5 or more pairs of arms arranged 

 about the mouth (Fig. 392), so named from Ke<pa.\'fi, head, and irovs, foot. Some, like the 

 Nautilus, have an external chambered shell, and others (Squids) only an internal bone or 

 pen. Bhy^icholites, sometimes found as fossils, are the hawkbill-like jaws of the species 

 of Ammonites. 



The subdivisions are : the Tetrabranchs, or 4-gilled species (Fig. 401), including the 



Nautili and Ammonites, and the 

 392. Dibrauchs, or 2-gilled species, 



which never have an external 

 chambered shell, and include the 



large Devil-fishes and the Argo- 

 naut, or Octopods ; the Cuttle- 

 fishes and Squids, or Decapods 

 (Fig. 392). In the latter group, 

 one pair of arms is very long, 

 and there is an internal horny 

 or calcareous bone (shell) some- 

 times called the pen (Fig. 392, p) 

 situated in the back. One spe- 

 cies of the Newfoundland seas has the body 15 feet long and the long arms about 35 feet. 

 The Sepia, from its ink-bag, affords the brown paint called sepia ; and its "pen" is the 

 spongy cuttle-fish bone used to supply lime in bird-cages. 



2. Pteropods. — Free-swimming species, having for the purpose of locomotion (Fig. 

 400), a pair of paddle-like plates near the head ; shell, when present, often slender, conical, 

 tliin, and glassy, but also of other shapes, and rarely spiral (Limacina). Named from 

 TTT^pSv, wing, and wovs. 



3. Scaphopods. — The foot adapted for burrowing. Shell tubular, conical, or oblong, 

 slender, as in Dentalium. Named from oKcirpos, digging, and irovs. 



4. Gastropods ( Cephalophora) . — Head prominent and furnished with eyes and 

 usually tentacles (Fig. 399); the mouth with a rasp-like tongue ; the foot, for locomotion, a 

 broad, flat, ventral surface, whence the name of the group (from yaarrip, the venter) ; shell, 

 a dorsal secretion, usually spiral, but in Chiton, a jointed symmetrical shield ; in some, 

 conical ; sometimes wanting. Includes the Snails (Fig. 399) among land species, and the 

 spiral shells of fresh and salt water, often called Univalves ; also species without shells, 

 some of which {Nudihranchs) have the gills in fllower-like groupings on the back. The 

 mantle varies much in extent, reaching (at the will of the animal) as far up the outside 

 of a shell as the surface is highly polished. Besides the eyes of the head, several species of 

 Naked Mollusks of the genus Onchidium have eyes over the back ; and these eyes, unlike 

 those of other Invertebrates, are like the eyes of Vertebrates in structure, a layer of rods 

 and cones forming the outer layer of the retina, and the general arrangement of the parts 

 being Vertebrate-like (Semper, Animal Life, 1881, page 371). 



5. Lamellibranchs (Figs. 396-398). — Include the Clam, Oyster, and other " bivalves." 

 They have no eye in the head portion, and no projecting head (whence called Acephals), 

 and no teeth or denticles in the mouth. The foot in many is a tough, keel-shaped, or 

 flattened muscular projection ; but sometimes it is small and spins horny fibers (byssus) 

 for attachment to rocks, and sometimes (as in Oysters, etc.) it is wanting. They have a 

 bivalve shell, the valves situated either side of the body, and articulated together above 

 between the iimbones. The valves show, inside, the impressions of one (at 2, Fig. 398) or 



