See also:

• LAMELLIBRANCHIA (Lat. lamella, a small or thin plate, and Gr. /3payxia, gills)

Both ctenidia, right and left, are invariably present, the See also:

• AXIS (Lat. for " axle ")

A pair of large glandular outgrowths, the so-called " See also:

• LIVER (O. Eng. lifer; cf. cognate forms, Dutch lever, Ger. Leber, Swed. lefver, &c.; the O. H. Ger. forms are libara, lipora, &c.; the Teut. word has been connected with Gr. i'prap and Lat. jecur)

the mantle margin. aa, Line of concrescence of the e, Incurrent siphonal notch of inner lamella of the right the mantle margin. inner gill-plate with the f, Foot. inner lamella of the left g, Probe passed into the inner gill-plate. See also:

• SUPERIOR

r, Left internal gill-plate. ar, Non-glandular portion of the re, Inner lamella of the right left nephridium, inner gill-plate. as, Anus. rg, Right See also:

• OUTER

The left side of the animal is seen as when removed from its shell in fig. i (I). The valves of the shell have been removed by severing their adhesions to the muscular areae h, i, k, 1, m, u. The free edge of the left See also:

• HALF

When the pedal aperture is small and far forward there may be a fourth aperture in the region of the See also:

• FUSION

The single adductor muscle of the Monomya is separated by a difference of fibre into two portions, but neither of these can be regarded as possibly representing the anterior adductor of the other Lamellibranchs. One of these portions is more liga- mentous and serves to keep the two shells See also:

• CON

The adductor muscles placed in the concavity of the shells act upon the long arms of the lever at a See also:

• MECHANICAL

These notches are known in Anodonta as the afferent and efferent siphonal notches respectively, and correspond to the long See also:

• TUBE (Lat. tuba)

the nacreous layer. The last is secreted by the whole surface of the mantle except the border, and additions to its thickness continue to be made through life. The periostracum is produced by the extreme edge of the mantle border, the prismatic layer by the part of the border within the edge. These two layers, therefore, when once formed cannot increase in thickness; as the mantle grows in extent its border passes beyond the formed parts of the two outer layers, and the latter are covered internally by a See also:

• DEPOSIT (Lat. depositurn, from deponere, to lay down, to put in the care of)

It can be pro- truded between the flaps of the mantle (fig. i [i] [2]) so as to issue from the shell, and by its action the Anodonta can slowly crawl or burrow in soft mud or sand. Other Lamelli- branchs may have a larger foot relatively than has Anodonta. In Arca it has a See also:

• SOLE (Solea)

Their appearance and position suggest that they are in some way related morphologically to the gill-plates, the anterior labial tentacle being a continuation of the outer gill :plate, and the posterior a continuation of the inner gill-plate. There is no embryolggical See also:

• EVIDENCE (Lat. evidentia, evideri, to appear clearly)

These processes are hollow, and receive the venous blood from, and return it again aerated into, the hollow axis, in which an afferent and an efferent blood-See also:

• VESSEL (O. Fr. vaissel, from a rare Lat. vascellum, dim. of vas, vase, urn)

(Mytilus) is represented, having e, f, Anterior portions of these axes precisely the same structure as fused by concrescence to the those of Arca. The filaments wall of the body. of the gill (ctenidium) of Mytilus g, Anterior adductor muscle. and Arca thus form two closely h, Posterior adductor. set rows which depend from the i, Anterior labial tentacle. axis of the gill like two parallel k, Posterior labial tentacle. plates. Further, their structure 1, Base line of the foot. is profoundly modified by the m, Sole of the foot. curious condition of the free n, Callosity. ends of the depending filaments. These are actually reflected at a See also:

• SHARP, GRANVILLE (1735-1813)
• SHARP, JAMES (1618-1679)
• SHARP, JOHN (1645-1714)
• SHARP, RICHARD (1759-1835)
• SHARP, WILLIAM (1749-1824)
• SHARP, WILLIAM (1856-1905)

Even in Mytilus and Area a connexion is here and there formed between the ascending and descending rami of a filament by hollow extensible outgrowths called " interlamellar junctions (il. j in B, fig. I I). Nevertheless the filament is a complete tube formed of chitinous substance and I clothed externally by ciliated epithelium, internally by endothelium and See also:

• LACUNAR

In the See also:

• DRAWING

o, Upper blood-vessel of the axis. n, See also:

• LOWER

The filaments take on a secondary grouping, the surface of the lamella being thrown into a series of half-cylindrical ridges, each consisting of ten or twenty filaments; a filament of much greater strength and thickness than the others may be placed between each pair of See also:

• GROUPS
• GROUPS, THEORY

16, A. The gill axis d is seen lying in the sub-pallial chamber between the foot b and the mantle c. From it depend the gill-filaments or lamellae—formed by united filaments—drawn as See also:

• BLACK
• BLACK, ADAM (1784-1874)
• BLACK, JEREMIAH SULLIVAN (1810–1883)
• BLACK, WILLIAM (1841-1898)

1, Labial Tentacle. l.a, Filamentous appendage of the labial tentacle. lb, See also:

• HOOD,
• HOOD, JOHN BELL (1831–1879)
• HOOD, SAMUEL HOOD, VISCOUNT (1724–1816)
• HOOD, SIR SAMUEL (1762-1814)
• HOOD, THOMAS (1799-1845)
• HOOD, TOM (1835–1874)

Dilated blood-space. Chitinous lining (compare A). p, C. A,Part of four filaments seen from the outer face in order to show the ciliated junctions c.j. B,Diagram of the posterior face of a single complete filament with descending ramus and ascending ramus ending in a See also:

• HOOK, JAMES CLARKE (1819-1907)
• HOOK, THEODORE EDWARD (1788-1841)
• HOOK, WALTER FARQUHAR (1798-1875)

al A ch c lac o.l (After R. H. Peck.) f, Constituent filaments. lac, Lacunar tissue. ch, Chitonous substance of the filament. chr, Chitonous rod embedded in the softer substance ch.' shown diagrammatically in fig. 16, C, and more correctly in fig. 17. In this region the inner lamellae of the inner gill-plates are no longer lamellar junction. The series of See also:

• OVAL (Lat. ovum, egg)

In the lateral view of the animal with reflected mantle-skirt and gill-plates, d the line of concrescence of the inner lamellae of the inner gill-plates is readily seen ; it is marked as in fig. 1 (5). In the same figure the free part of the inner lamella of the inner FIG. 15.—Diagram of a view from gill-plate resting on the foot the left side of the animal of Anodonta is marked z, whilst the at- cygnaea, from which the mantle-skirt, t a c h e d p a r;t—the most the labial tentacles and the gill-filaanterior—has been snipped ments have been entirely removed so with See also:

• SCISSORS

,eImm~ f A, Outer gill-plate. B, Inner gill-plate. C, A portion of B more highly o.l, Outer lamella. [magnified. Inner lamella. v, Blood-vessel. Diagram of a See also:

• BLOCK
• BLOCK (from the Fr. bloc, and possibly connected with an Old Ger. Block, obstruction, cf. " baulk ")
• BLOCK, MAURICE (1816–19or)

r, Line of origin of the posterior labial tentacle. by the excurrent or superior siphonal notch of the mantle (fig. I, d); the lower space communicates by the lower siphonal a fr f- ft f A, Shows two conditions with er, Reflected lamella of outer gill- f ree gill-axis. plate. B, Condition at foremost region f, Adaxial lamella of inner gill-in A nodonta. [donta. plate. C, See also:

• HIND

A probe (g) is introduced through this slit-like passage, and it is seen to pass out by the excurrent siphonal notch. It is through this passage, or in-directly through the pores of the gill-plates, that the water introduced into the lower subpallial space must pass on its way to the excurrent siphonal notch. Such a subdivision of the pallial chamber, and direction of the currents set up within it do not exist in a number of Lamellibranchs which have the gill-lamellae comparatively free (Mytilus, Arca, Trigonia, &c.), and it is in these forms that m, Mantle-flap. lamellah. br, Outer, b'r', inner gill-plate—each In the 9th edition of this composed of two lamellae. See also:

• ENCYCLOPAEDIA

The coil of the intestine in Anodonta is similar to that of other Lamellibranchs. The rectum traverses the pericardium, and has the ventricle of the heart wrapped, as it were, around it. This is not an unusual arrangement in Lamellibranchs, and a similar disposition occurs in some Gastropoda (Haliotis). A pair of ducts (ai) See also:

• LEAD
• LEAD (pronounced iced)

and of two auricles which open into the ventricle by orifices protected by valves. The blood is colourless, and has colourless amoeboid corpuscles floating in it. In Ceratisolen legumen, various species of Arca and a few other species the blood is See also:

• CRIMSON

c, Dorsal wall of the pericardium cut and reflected. See also:

• RENO

Each nephridium, after being See also:

• BENT
• BENT (E1. BENI)
• BENT, JAMES THEODORE (1852–1897)

Hence, except for the formation of a urino-genital groove, the apertures are placed as they are in Anodonta. Previously to Hoek's See also:

• DISCOVERY

Posteriorly beneath the posterior adductors, and covered only by a thin layer of elongated epidermal cells, are the visceral ganglia. United with these ganglia on the outer sides are the osphradial ganglia, above which the epithelium is modified to form a pair of sense-organs, corresponding to the osphradia of other Molluscs. In some Lamellibranchs the osphradial ganglia receive nerve-fibres, not from the visceral ganglia, but from the cerebral ganglia along the visceral commissure. Formerly the posterior pair of ganglia were identified as simply the osphradial ganglia, and the anterior pair as the cerebral, pleural and visceral ganglia united into a single pair. But it has since been discovered that in the Protobranchia the cerebral ganglia and the pleural are distinct, each giving origin to its own connective which runs to the pedal ganglion. The cerebro-lower half of their length, or for nearly the whole length. Moreover, in many forms, in which in the adult condition there is only a single pair of anterior ganglia and a single pedal connective, a pleural ganglion distinct from the cerebral has been recognized in the course of development. There is, however, no evidence of the See also:

• UNION
• UNION (known locally as Union Hill and officially as Town of Union)

A single otolith is present as in the veliger embryos of Opisthobranchia. In Filibranchia and many Protobranchia the otocyst (or statocyst) contains numerous particles (otoconia). The organs are developed as invaginations of the epidermis of the foot, and in the See also:

• MAJORITY (Fr. majorite; Med. Lat. majoritas; Lat. major, greater)

They are totally distinct from the cephalic eyes of typical Mollusca, and have a different structure and See also:

• HISTORICAL

a.ad,Anterior adductor. p.ad,Posterior. adductor. mt, Mantle-flap. bFoot. r, Branchial filaments. au.v, Otocyst. at, Alimentary canal. eye. It must, however, be distinctly borne in mind that there is a fundamental difference between the eye of Vertebrates and of all other groups in the fact that in the See also:

• VERTEBRATA

A new aperture, m, the mouth, has N.B.—In this development, as in that of Pisidium (fig. 25), no part of the blastopore persists either as mouth or as anus, but the aperture closes—the pedicle of invagination, or narrow See also:

• NECK (O. Eng. hnecca; the word appears in many Teutonic languages; cf. Dutch ride, Ger. Nacken; in O. E. the common word was heals; cf. Ger. Hals)

Generative Organs.—The gonads of Anodonta are placed in distinct male and See also:

• FEMALE

The blastopore has open to the exterior by two simple ;closed. ducts, one right and one left, continu- ous with the tubular branches of the gonads. In the most primitive Lamellibranchs there is no separate generative aperture but the gonads discharge into the renal cavity, as in Patella among Gastropods. This is the case in the Protobranchia, e.g. Solenomya, in which the gonad opens into the reno-pericardial duct. But the generative products do not pass through the whole length of'the renal tube: there is a See also:

• DIRECT

22). The glochidium quits the gill-pouch of its parent and swims by alternate opening and shutting of the valves of its shell, as do adult Pecten and Lima, trailing at the same time a long byssus See also:

• THREAD (0. Eng. praed, literally, that which is twisted, prawan, to twist, to throw, cf. " throwster," a silk-winder, Ger. drehen, to twist, turn, Du. draad, Ger. Draht, thread, wire)

tp, Velar area of the prostomium. The extent of the shell and commencing upgrowth of the mantle-skirt is indicated by a line forming a See also:

• CURVE (Lat. curvus, bent)

The gastrula is formed in this case by invagination. The embryonic cells continue to See also:

• DIVIDE

A later stage is seen in fig. 26, where the pharynx is widely open and the foot prominent. No ciliated velum or pre-oral (cephalic) See also:

• LOBE

In Nucula delphinodonta the test is uniformly covered with short cilia, and there is no flagellum. When the larval development is completed the test is See also:

• CAST (from the verb meaning " to throw "; the word is Scand. in origin, cf. Dan. kaste, and Swed. kasta; " cast " in Middle Eng. took the place of the A.S. weor pan, cf. Ger. werf en)