Page:The American Cyclopædia (1879) Volume XI.djvu/741

 MOLLUSCA 723 they are papillose and arranged along the sides of the back. III. Inferobranchiata, like phyl- lidia, in which the branchise are at the lower part of the sides of the body. IV. Tectibran- chiata, in which the leaf-like branchiae are covered by the mantle and a small shell ; as in aplysia (sea hares), formerly dreaded on ac- count of their strange form, and the violet fluid they eject when molested, in umbrella, and in 'bulla (bubble shell). V. Pulmonata, in which a part of the mantle cavity forms a vas- cular air sac or lung ; most are terrestrial, and such as live in the water rise to the surface to breathe ; a few are naked, but most are shell- bearing, without or with an operculum ; in the inopercnlated, with a well developed shell, are helix (snails), succinea (amber snail),vitrma, bu- limus, pupa, achatina, and other land snails, the slugs (Umax), land soles (arion), pond snails (limnea), &c. ; in the operculated are cyclostoma, helicina, acicula, &c. In the dioecious gastero- pods belong the following four divisions : I. Tu- bulibranchiata, in which the branchiae are two, symmetrical, behind the heart, and enclosed with the other soft parts in a long shelly tube ; as in dentalium (tooth shells). II. Cyclobran- chiata, in which the branchiae are a series of lamellae, surrounding the body between the foot and mantle ; as the limpets (patella) used as food and for bait, and the sea wood-lice (chiton), with multivalve shell pieces like the carapace of articulates. III. Dentibranchiata, in which the branchiae are plumose or pecti- nate, and with the body protected by a widely opened inoperculate shell ; as in the ear shells (haliotis) ; the delicate violet shells (janthina), found abundantly in mid ocean, feeding upon the acalephan velellm, and suspended by a raft of air vesicles, to the under surface of which the egg capsules are attached ; and thejissurel- la, or key-hole limpets. IV. Pectinibranchia- ta, in which the two comb -like branchiae are contained in a dorsal cavity of the mantle opening widely above the head ; they have two feelers and two eyes, and a proboscis capable of elongation in a tube form ; the females se- crete an albuminous matter in which the eggs are enveloped, a familiar example being the yel- low grape-like bunches of the whelk (bucci- num). Here belong the bonnet limpet (calyp- trcea) and the slipper shell (crepidula) ; the top shells (turbo), and the pheasant shells (phasia- nelld) ; the river snails (paludina), the peri- winkles (litorina), the turret shells (turritella the wentletraps (scalaria), the cerithium, and natica; cowries (cyprced), very handsome shells, and one species, C. moneta, used as money on the "W. coast of Africa ; marginella, valuta, mitra (mitre shells) ; the tuns (dolium), harps (harpa), whelks (buccinum) ; rock shells (mu- rex), fig shells (pyrula), wing shells (strombus), the seraphs (terebellum), and numerous others. The class of cephalopoda, the highest type of mollusks, is characterized by the locomotive and prehensile organs being attached to the head, whence they radiate in the form of mus- cular arms and tentacles, and by an internal skeleton combined in some with an external shell, though the integument in most is uncal- cified^and flexible; the head is free and the body is covered by a muscular sac or mantle, with a transverse anterior aperture, from which projects the expiratory siphon or tube ; the branchiae are concealed, the sexes distinct, and the animals oviparous, aquatic, marine, preda- tory and carnivorous, nocturnal, and social; the colors are changeable and brilliant; they emit an inky secretion when disturbed, which permits them to escape by the discoloration of the water ; this is what the true India ink is made from. This class have a rudimentary internal skeleton ; in the head of most is a car- tilaginous ring around the oesophagus, the up- per part covering the cerebral ganglion, and containing the organs of vision and hearing ; there is often an additional cartilage to which the muscles of the arms are attached, and oth- ers on the back and sides. The mouth is in the middle between the arms, and has two jaws like the bill of a parrot, the lower the larger ; the head is separated from the body by a con- striction like a neck ; there is a well marked tongue. The sexual organs are at the base of the visceral sac, and the spermatophores are very active ; in some of the octopods, one of the arms is deciduous, and becomes a male or- gan, described by Cuvier as hectocotylus and a parasite ; the eggs are laid in heaps or bunches, attached to each other and to foreign bodies. The nervous system is largely developed. For respiration water is drawn in and expelled by the muscular action of the mantle and funnel, as the gills have no vibratile cilia ; the water enters the branchial cavity at the anterior open- ing of the mantle, and is forced out through the funnel, propelling the animal backward. In the first order, the tetrdbranchiata, the branchiae are in two pairs, without branchial hearts, and the mantle is thin and not very muscular ; the ink bag is absent ; the arms are very numerous, hollow, and with retractile tentacles ; eyes pedunculate ; the head retrac- tile within a many-chambered siphunculated cell. Among existing mollusks this order con- tains only the genus nautilus (see NAUTILUS) ; in past ages lived the ammonites, baculites, hamites, orthoceratites, turrilites, &c. In the second order, the dibranchiata, the branchiae are two, each with a branchial heart ; the fun- nel is an entire tube, and the mantle is muscu- lar ; an ink bag is present ; there are eight non- retractile arms, large and complicated, bearing sucking disks or acetdbula, with usually two additional long arms ; the eyes are sessile and in orbits ; the shell is internal, except in the female argonauts. In the decapod tribe, with eight arms and two tentacles, belong the genus spirula, the extinct belemnites, the cuttle fishes (sepia), and the squids. (See SQUID.) In the octopod or eight-armed tribe there are no tentacles, the arms have sessile suckers, and the branchial chamber is divided by a longitu-