MOLLUSCA 



127 



axis, and retain, both externally and in the disposition of 

 internal organs, the archi-Molluscan BILATERAL 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 

 FOEM 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] 



FIG. 74. Diagrams of the anatomy of Dentalinm. 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 ; l>, anterior free margin of the same ; c, hinder extension of 

 tin- mantle-skirt; (i, the appendix of the mantle-skirt separated liyavalve 

 from the peri-anal portion of the sub-pallia! chamber, h ; c, the snout or oral 

 process ; f, the foot ; (i, the ctenidial filaments ; h, the peri-anal part of the 

 sub-pallia! chamber ; ?", the peri-oral part of the same chamber ; /,-, the anus ; 

 /, the left nephridium ; ?, the mouth surrounded by pinnate tentacles ; n, 

 the bnccal mass and odontophore ; o, oesophagus; p, the left lobe of the 

 liver; g.p, pedal ganglion-pair; f/.c, cerebral ganglion-pair; g.pl, plenrul 

 ganglion-pair; g.v, visceral ganglion-pair. Possibly further research will 

 show that >i.p! is the typieal viseeral ganglion-pair, and that g.v is a pair 'it 

 olfactory ganglia placed on the visceral loop as in the Lipocepnala according 

 to Spengel. 



placed at the base of the cylindrical cephalic prominence 

 or snout (fig. 74, e). A pair of NEPHEIDIA (fig. 74, I) are 

 present, opening near the anus (fig. 74, /). The right 

 serves as a genital duct, the left is apparently renal in 

 function. The LIVER (;)) 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 pairs, the typical pleural pair being closely 

 joined to the cerebral. The visceral loop or commissure is 

 untwisted, that is to say, the Seaphopoda 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 : Dentnlium, L. (figs. 

 73, 74) ; Siphonodentalium, Sars. ; and Entaiium, 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-, 



FIG. 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 



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- 



