Page:The American Cyclopædia (1879) Volume XI.djvu/742

724 MOLLUSCOIDS dinal partition ; the arms are more robust, and are often united by a web at the base, con- stituting a powerful swimming organ. Among the naked octopods belong the so-called sea spiders. (See OCTOPUS.) Eledone and tre- moctopus are allied genera. The genus argo- nauta or paper nautilus is well known for the delicate and beautiful shell of the female. (See NAUTILUS.) The shell is used only for protect- ing and hatching the eggs ; the male has no shell, and impregnation is effected by a decid- uous Jiectocotylus. The local distribution of f aunee and the distinctness of zoological regions are well illustrated by mollusks ; while some are very limited in their range, others, like the cyprcea, are extensively spread even across ocean barriers ; some are cosmopolite, wander- ing wherever their food is found ; helix cella- ria, attaching itself to water casks, occurs in most seaports of the world, H. similaris wher- ever the coffee plant grows, and If. mtrinoides follows the taro or arum esculentum. As a general rule, according to Mr. Jeffreys, speci- mens are larger toward the north than toward the south ; colors are usually the brightest in the tropical seas, except in specimens from great depths. (For details on distribution, see " Mollusca and Shells of the United States Ex- ploring Expedition," 1838-'42, by A. A. Gould, M. D., Boston, 1852.) The distribution of mol- lusks in time extends from the lower Silurian to the present epoch ; all the classes are repre- sented in the earliest fossiliferous strata; some families, like the ammonites and belemnites, have passed away; others, like the nautilus, are verging toward extinction ; some have continued with slight specific modifications from the Silurian to the present day. Lamel- libranchiate have succeeded palliobranchiate bivalves ; siphonate have succeeded asiphonate univalves ; and the dibranchiate now vastly outnumber the tetrabranchiate cephalopods. Whole strata of the earth's crust are made up principally of the shells of mollusks. Mol- lusks supply an abundant, wholesome, and usually easily digestible article of food to na- tions civilized and savage, as well as to other animals; bivalves are considered the best, as having the least muscular fibre. The orna- mental purposes to which the pearl and cameo shells are put are well known ; from the cuttle fish is obtained sepia and India ink ; from the purpura and ~buccinum of the Mediterranean came the famous Tyrian dye of antiquity ; from the filaments of the byssus of pinna are made tissues much esteemed on the shores of the Mediterranean. On the other hand, mol- lusks are sometimes injurious to man; slugs and snails do mischief in gardens ; the teredo pierces ship timber, and the pholas bores into and weakens stone dikes. The number of species of mollusks probably exceeds 25,000, surpassed only by the number of articulates. MOLLIJSCOIDS, a division of the old branch of mollusca, first made by Milne-Edwards to in- clude the ~bryozoa and ascidians or tunicates, to which have since been added the brachio- pods ; all of which are now regarded by Prof. E. S. Morse and others as articulates, having certain molluscan affinities, but coming nearest to the tubicolous worms. (See BRACHIOPODA, and BEYOZOA.) In the lowest of these, the Iryozoa, are comprised small pedunculated ani- mals, the margin of whose body is provided with vibratile cilia, for producing the water currents necessary to respiration and to the obtaining of food; these cilia are sometimes supported on long tentacular prolongations; the digestive cavity is distinct from the walls of the body, and can be traced as a canal from mouth to vent, both opening within the ciliated circle, being reflected upward ; they propagate by buds and by free swimming ciliated gem- mules. They seem to have both males and females on the same stem, the cells containing animals with eggs being apparently more nu- merous than those with spermatozoa ; the mus- cular system is largely developed, and serves principally to retract the animal within its cell. They have been divided by Van der Hoeven into the families : 1, stelmatopoda, in which the tentacles are disposed in a zone around the mouth, as in the genera escJiara, flustra, and cellularia ; and 2, lophopoda, with tenta- cles set pectinately on two arms, and numerous, such as cristatella, plumatella, and alcyonella. The tunicata, including the ascidians and salpcs, have no shells, but are enclosed by an elastic, cellulose, uncalcified integument, having two apertures; the circulation is peculiar in the phenomenon of venous blood at one time pro- ceeding from the heart to the gills, and at an- other arterial blood from the gills to the heart, in the same vessels; respiration is effected either by a vascular ciliated pharyngeal sac, or by a ribbon-shaped gill stretched across the common visceral cavity; the nervous system presents a single ganglion, from which the nerves radiate; organs of feeling, sight, and even hearing, have been described in these animals ; muscular fibres, both longitudinal and transverse, are well developed. In salpce we have free swimming animals, drawing in water by one aperture and expelling it by an- other one opposite ; they are numerous in the Mediterranean and in the temperate parts of the ocean far from the shores, and are said to be phosphorescent at night; they sometimes occur singly, and sometimes in long chains or in rings ; Ohamisso concluded, from observing the living animals, that a generation of distinct salpce alternates with one of those in a chain ; within the single individuals connected embryos were found, which, with other similar phenom- ena, led to the interesting work of Steenstrup on the " Alternations of Generation ;" the solitary salpce are sexless, and are propagated by internal germs or buds, and are inferior to the associated forms, which have reproduc- tive organs; the latter produce each a single young one. The ascidians have a sac-like body, with two apertures generally near together;