Page:The American Cyclopædia (1879) Volume XI.djvu/740

 722 MOLLUSCA holy water, sometimes 2 ft. wide, genus tri- dacna, the largest of the class), the horse-foot clam (hippopus), the edible quahaug (vemts mercenaries, the shell from which the wampum of the American Indians was made), the small fresh-water cyclas, the common clam (mya are- naria), the razor shell (soleri), the pholas (pid- dock or stone-borer), the ship worm (teredo), so destructive to timber in vessels and dock- yards, the waterpot shells (aspergilluni), and the club shells (clavagella). All bivalves are very prolific ; in those which, like the oyster, are fixed, the sperm cells of the male are car- ried by the currents of the water to the cavity of the mantle of the female. The remaining three fourths of mollusks are called encephala, from having a distinct head, commonly with eyes and tentacles, and a mouth with a com- plex masticatory apparatus ; they have been divided into the classes of cephalopJiora (head bearers) and cephalopoda (with the head sur- rounded by the feet). The cephalopJiora have been subdivided, according to the- modifications of the locomotive organs, into the orders of pteropoda, heteropoda, and gasteropoda. The pteropoda are so called from two wing-like muscular expansions from the sides of the an- terior part of the body, used as swimming or- gans, and not, according to Owen, homologous with the foot of gasteropods ; they are small, marine, floating, hermaphrodite, and oviparous; the form is very variable, some being globular, others long and slender ; the heart, as in the whole class, is arterial ; the urinary sac, within the mantle and near the heart, communicates with the respiratory cavity and with the peri- cardial sinus, introducing water into the blood ; some are naked, others are provided with very delicate shells of various forms ; the eyes are not well developed, but the acoustic sac exists in all ; the naked species have four tentacles, the testaceous ones two. In the family ihe- cosomata, the head is indistinct, and the shell fragile ; the best known genera of this family are hyalea and cleodora, found in the warmer temperate and tropical seas ; some of them are beautiful objects, as they swim through the water like butterflies in the air ; one of the largest and finest is the H. tridentata, three fourths of an inch long, commonly known as the " chariot of Venus." In the family gym- nosomata, or naked pteropods, the head is dis- tinct, and the fins are attached to the sides of the neck ; it includes the genera clio and pneu- modermon ; of the former, the C. lorealis ex- ists in such immense numbers in high northern latitudes, that it forms a chief portion of the food of the Greenland whale, and is hence called "whale bait" by the fishermen; it is hardly an inch long. The order heteropoda is characterized by a compressed fin-like foot having a suctorial disk; the branchifiB are fringed or pinnate ; the sexes are distinct. All are marine, and usually are rapid swimmers with the back downward and the foot upward ; the foot corresponds to the anterior portion of this organ in gasteropods. They are some- times called nucleobranchiates, and may be di- vided into the families atlantidce and firolidce. In the first family belongs the atlanta, with a delicate shell large enough to protect the body, found in great numbers in the midst of the tropical and temperate oceans; in these the foot supports the operculum. In the second family is placed carinaria, sometimes called the "glassy sailor," which has an elongated body, with a very small keeled shell at the pos- terior part, the apex turned backward ; on the head are two long tentacles, and two ses- sile eyes behind their base ; the middle part of the foot is reduced to a compressed fin-shaped lobe, with a small suctorial disk, by which they adhere to seaweeds, &c. ; their motions are rapid and graceful, and they inhabit the tem- perate and tropical waters; a small species is found in the Mediterranean ; the shell of the G. vitrea, from the Indian ocean, is highly prized. In the genus firola or pterotrachea there is no shell, and the animal is almost transparent; there are two eyes, and generally no tentacles, but a slight fleshy proboscis ; they swim or float free in mid ocean in great numbers, and also in the Mediterranean. In the order gas- teropoda there is a large muscular disk for creeping developed from the ventral surface of the body (hence the name), as in the com- mon slugs and snails. They are usually un- symmetrical, the visceral portion of the body coiled spirally and protected by a univalve shell, the organs of respiration being generally atrophied; the shell is almost always closed by a calcareous, horny, or albuminous oper- culum. Most of them are marine, some in- habit fresh water, and a few are terrestrial ; they have been divided according to the char- acters of the breathing apparatus. In some (moncecia) the male and female organs are in the same individual, in others (diacia) the sexes are distinct; most are oviparous, but a few (certain snails) are ovoviviparous. In the water breathers the young are excluded with an operculated shell, which in the naked spe- cies is either shed or concealed by the mantle, and by means of ciliated fins on the sides of the head they move far away from their inac- tive parents, undergoing several metamorpho- ses in the process of growth ; the air breath- ers pass through no such changes. They have the power of repairing injuries and of repro- ducing lost parts to a considerable degree. Among the monoecious gasteropods are the following five divisions : I. Apneusta, having no distinct respiratory organs, but in their place an extensive aquiferous system, and no shell in the adult ; the body is soft and elonga- ted, the integument ciliated ; they are marine ; calliopaa and actceon are well known genera. II. Nudibranchiata, with the branchiae extend- ing freely from various parts of the body, as in glaucus, doris (sea lemons), in which the branchiae form a plume-like circle in the mid- dle of the back, and eolis (sea slugs), in which