MOLLER. 691 MOLLXJSK. pul-Hc liviililiiios and private resiliences at Darm- stadt, and the theatre at Mainz, lie diseuvered the original design of the Cologne Cathedral, the two towers of which have been finished in accordance with his published facsimile. His most important publications include Dt-itk- miiicr detitscher liaukunst (181.5-51) and Bcitriige xti dcr Lehre von den Koiialiulctioncn (1833-44). MOLLHAUSEN, mel'hou-zcn, Baldii.n (1825 — ), A German traveler and novelist, born at Bonn. He studied agriculture in I'onicrania, and spent several years in North .mcrica. travel- ing with Duke Paul of Wurttembcrg ( 1851 ) , and, at the instance of Humboldt, acting as draughts- man to a scientific expedition to the far est. After a second trip through the Western part of the United States, especially Colorado, he wrote Tayehuch einrr Reise ^^om ilissi/isijipi nach den Kiisien der Siidsee (1858) and Reisen in die Felsengebirge yordantcrikus (IS(il). His many novels, mostly on American topics, include: Die Ilalbiiuliaiter 11861); Ikis Mornioneiimiidrhen (1804: 3d ed. 1871); Das Monogriimm (1874); Die heiilrn Yachien (1891); and Fegefeuer in Fraiiiirs Wigicain (1901). MOLLITIES (mol-lishl-ez) OS'SIUM. See iOsTEOMAL.CIA. MOLLTJS'CA (Lat. nom. pi. neu. of moll us- Ills, from inoJIis. soft). One of the chief divi- sions, or phyla, of the animal kingdom, the study i>i which is Conchology or JIalacologj-. The liody is bilaterally symmetrical except in snails (Gastropoda), not segmented as in worms and arthropods, but soft, fleshy, and usually protected hy a bivalve or univalve shell : moving by a ■foot" or muscular creeping disk in gastropods, or, in bivalves, by a tongue-like process; breath- ing by external gills which are either lamellate or plume-like. A shell is secreted by the fleshy mantle, and in nearly all except the Pelecypoda (q.v.), which are headless, the mouth is armed with an 'odontophore,' an ai>paratus of muscles and tendons bearing a rasp-like "lingual ribbon' (radnla) for sawing or cutting the food, or for drilling holes through shells, ilany mollusca in their young or larval swimming stages begin as a "troehosphere' (q.v.), pass through a 'veliger' stage, living at the surface of the sea and gradu- ally sinking to the bottom by gravity as the shell grows larger and heavier. The ilollusca form a highly specialized group, the number of species amounting to upward of 40.000, about one-half of which are living, the other extinct; the eai'liest known species occur in Cambrian rocks. The grou]> lias a wide geographical and bathymetrical range, occurring in all seas from the shore to the abysses of the tu-can and also on land and ill fresh waters. The allinities of this phylum are not yet .settled. Mollusks have evidently descended from some worm-like form, as their larva> are in some cases segmented and like the troehosphere and 'cejihalula" stage of sea-worms (. nelida ) . In the ailult segmented organs, like those of worms, arc often present. On the whole they are a grade inferior to the spiders and insects. Jlollusks are divided into five classes, i.e. : ( 1 ) Amphineura ; A beg.men'ted larva. Trochoephere of Chiton. (2) Pelecypoda (q.v.) or Laniellibranchiata ; (3) Seaphopoda (q.v.); (4) Gastro])oda (q.v); and (5) Cephalojioda (q.v.). The Amphineura com- prise a few forms formerly supposed to be worms (Cha'toderma and Nconienia), These are primi- tive types from which, as some authors think, the other mollusks may have descended; all have a ladder-like nervous system, as in Chiton, which also has a segmented larva, and are cither shell- less or somewhat like that of some Turbellaria (<l.v.) and Peripatus (q.v.). See ^iloLLUSK. MOLLUSK. An animal of the phylum Mol- lusca (q.v.). JloUnsks are usually easy to dis- tinguish from other animals on account of their shell, whence they are commonly called 'shell- fish'; but the more we study their development and morphology the more dilficult is it to draw a definite line between them and certain worm- like forms. In their early development they travel along apparently the same developmental path as the worms (planarians as well as an- nelids), and then diverge into a separate path. That the type is a very successful one is jiroved by the enormous number of species both living and extinct — a success evidently due to the pro- tection afl'orded them by their shell. In bivalves as well as univalves (CJastropoda) the shell is more or less solid, is composed mainly of carbon- ate of lime and secreted by the mantle. A bivalve mollusk like the clam is completely protected by a pair of solid calcareous shells con- nected by a hinge consisting of a large tooth (in most bivalves there are three teeth) and liga- ment. The shells are equivalve or with both valves alike, but unlike at each end, the head end lieing more rounded. On the interior are two muscular impressions or 'scars' made by the two TRANSVERRE BECTIONS OF BIVALVE MOLLTTSKS. A clam (Mya) and a frpBb-water imisscl (Unio), Hhowing the ligament (L) aud adductor muscle (M). adductor muscles. The shell of gastropods is spiral, and that part of the animal contained in the .skin is asymmetrical, the twist or torsion be- ing due to gravity or lopping over of the young shell in the larva stage. As to the most primitive forms we are in the dark. The most characteristic mollusks are the cephalopods. They are bilateral and very highly modified, with a well marked head, containing two highly specialized eyes and two ears. The concentrated ganglia form a brain with cartila- ginous protections, while the parts around the mouth are modified to form the tentacles and funnel. The heart and blood vessels are highly specialized. The 'foot' is a modification of the part of the mantle below and behind the mouth: it varies greatly in shape, and is by disuse wanting in oysters and other fixed forms ; in the pclecypods or bivalves it is tongue-shaped, and by being filled
