Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/692

Rh 664 MOLLUSCA [CEPHALOPODA. axis, and retain, both externally and in the disposition of internal organs, the archi-Molluscan BILATEEAL SYMMETRY. The margins of the mantle-skirt of opposite sides (right and left) meet below the foot and fuse by concrescence ; only a small extent in front and a small extent behind of the mantle-margin is left unfused. Thus a CYLINDRICAL FORM is attained by the mantle, and on its surface a TUBU LAR shell (incomplete along the ventral line in the youngest stages) is secreted (fig. 73, D). The FOOT is greatly elon gated, and can be protruded from the anterior mantle- aperture. It has a characteristic clavate form (fig. 74, /). The pair of typical CTENIDIA are symmetrically deve loped in the form of numerous gill-filaments (fig. 74, A, g) Fio. 74. Diagrams of the anatomy of Dentalium. A. The anterior portion of the tubular mantle is slit open along the median dorsal line, and its cut margins (a) reflected so as to expose the foot, snout, and gills. B. Lateral view with organs showing as though by transparency. C. Similar lateral view to show the number and position of the nerve-ganglia and cords, a, the mantle-skirt ; 6, anterior free margin of the same ; c, hinder extension of the mantle-skirt ; d, the appendix of the mantle-skirt separated by a valve from the peri-anal portion of the sub-pallial chamber, h ; e, the snout or oral process ; f, the foot ; g, the ctenidial filaments ; h, the peri-anal part of the sub-pallial chamber ; i, the peri-oral part of the same chamber ; fc, the anus ; /, the left nephridium ; m, the mouth surrounded by pinnate tentacles ; n, the buccal mass and odontophore ; o, oesophagus ; p, the left lobe of the liver; g.p, pedal ganglion-pair; g.c, cerebral ganglion-pair; g.pl, pleural ganglion-pair; g.v, visceral ganglion-pair. Possibly further research will show that g.pl is the typical visceral ganglion-pair, and that g.v is a pair of olfactory ganglia placed on the visceral loop as in the Lipocephala according to Spengel. I laced at the base of the cylindrical cephalic prominence or snout (fig. 74, e). A pair of KEPHRIDIA (fig. 74, t) are present, opening near the anus (fig. 74, k). The right serves as a genital duct, the left is apparently renal in function. The LIVER (p) is large and bilobed, the lobes divided into parallel lobules. The NERVE-GANGLIA are present (fig. 74, C) as well-marked cerebral, pleural, pedal, and visceral pair,&quot; the typical pleural pair being closely joined to the cerebral. The visceral loop or commissure is untwisted, that is to say, the Scaphopoda are EUTHYNEUR- ous. HEART and distinct VESSELS are not developed ; a colourless blood is contained in the sinuses and networks formed by the body-cavity. The GONADS are either male or female, the sexes being distinct. ^ The embyro is remarkable for developing five ciliated rings posterior to the ciliated ring and tuft characteristic of the trochosphere larval condition of Molluscs generally. These rings are comparable to those of the larva of Pneu- modermon (fig. 84), and like them disappear. The class Scaphopoda is not divisible into orders or families. It contains only three genera : Dentalium, L. (figs. 73, 74) ; Siphonodenlalium, Sars. ; and Entalium, Dfr. They inhabit exclusively the sand on the sea-coast in depths of from 10 to 100 fathoms. It is worthy of remark that the Scaphopoda constitute among the Glossophora a parallel to the sand-boring forms so common among the Lipocephala (such as Solen and Mya). This parallelism is seen in the special mode of elongation of the body, in the form of the foot, and in the tubular form of the mantle brought about by the concrescence of its ventral margins, as in the Lipocephala mentioned. The cylindrical shell of Dentalium is also comparable to the two semi-cylindrical valves of the shell of Solen ; or, better, to the tubular shell of Aspergillum and Teredo. Nevertheless, it is necessary to consider the Scaphopoda as standing far apart from the Lipocephala, and as having no special genetic but only a homoplastic relationship to them, in consequence of their possessing a well-developed odonto phore, the characteristic organ of the Glossophora never possessed by any Lipocephala. Class III. CEPHALOPODA. Characters. Mollusca Glossophora with the FOOT prim arily adapted to a FREE-SWIMMING mode of life. The archi-Molluscan BILATERAL SYMMETRY predominates both in the external and internal organs generally, though in many cases (especially the smaller forms) a one-sided dis placement of primitively median organs and a suppression of one of the primitively paired organs is to be noted. An ANTERIOR, MEDIAN, and POSTERIOR region of the FOOT can be distinguished (fig. 75, (4), (5), (6)), corre sponding to but probably not derived from the pro-, meso-, y (i) (2) Fio. 75. Diagrams of a series of Molluscs to show the form of the foot and its regions, and the relation of the visceral hump to the antero-posterior and dorso-ventral axes. (1) A Chiton. (2) A Lamellibranch. (3) An Anisopleur- hecosomatous Pteropod. (5) A Gymnosomatous and meta-podium of Gastropoda. The fore-foot invariably has the HEAD MERGED into it, and grows up on each side (right and left) of that part so as to surround the mouth, the two upgrowths of the fore-foot meeting on the dorsal aspect of the snout, whence the name Cephalopoda. In the more typical forms of both branches of the class, the peri-oral portion of the foot is drawn out into paired arm-