Dissected to show the nervous system and the route along which the ova pass from the generative gland into the interlamellar cavity of the external gills, where, as in the pouch of a marsupial mammal, they are lodged, and go through certain stages of their development.

PART of the foot has been removed on the animal's left side to show the pedal ganglia in situ; the union of the inner lamellae of the inner gills behind the foot has been divided, and the glandular portion of the nephridium opened to show the nerve cord uniting the left cerebro-pleural to the left visceral ganglion in its entire length.

The left cerebro-pleural ganglion is seen lying upon the tendon of the retractor pedis anterior muscle, and just anteriorly to that of the protractor pedis muscle. It is connected to the ganglion of the same name on the right side by a commissure passing above the mouth, under which a slip of blue paper has been placed. It is united by a connective which passes obliquely to the left hand with the pedal ganglion of its own side. The right and left pedal ganglia are united closely inter se. They lie within the periphery of the 'visceral mass,' a division of the body which in these Molluscs is less sharply marked off from the 'foot' proper than in many other members of the phylum. Four or five delicate nerves may be seen passing off from the ganglion into the muscular portion of the foot. The auditory vesicle may be found appended close to the line limiting the viscera from their muscular envelope. Its nerve in Anodonta is derived from the cerebro-pedal connective (infra). It is not seen here.

A second connective passes backwards (downwards here) to the visceral ganglion, which lies upon the inferior surface of the posterior adductor muscle. It passes, first, between the fibres of the anterior retractor and of the protractor pedis muscles; and secondly, after skirting the inner edge of the orifice of the reproductive gland, through the glandular portion of the nephridium externally to the common tendon of the posterior retractor pedis muscles. The corresponding connective of the other ganglion, i. e. that of the right side, comes into view in front of as well as behind this last mentioned tendon. A slip of blue paper has been passed under both cords just before they enter their respective halves of the ganglion. The visceral ganglion is really paired, but its two parts are closely united in the median line. It gives off posteriorly to the right and left a stout pallial nerve which skirts the mantle. Similar nerves are given off by the cerebro-pleural ganglia, but are not seen in this preparation. Both sets of nerves unite in a circumpallial plexus with ganglia here and there. Other nerves (not visible) pass to the anus which lies in the median line behind the posterior adductor muscle, and to this muscle itself.

Anteriorly the ganglion gives off two stout nerves right and left to the gills. These nerves are beset in reality with ganglion cells, and are in relation with a modified epithelium. The whole represents the osphradium or olfactory apparatus of Spengel. There are no ganglia in a Lamelli-branch corresponding to the buccal ganglia of the Snail; but ganglia may be developed along the course of the pallial nerves, and on the siphonal nerves in those genera where siphons are well developed.

The lamellae of the glandular portion of the nephridium, the inter-lamellar space of the outer left gill, its interlamellar and interfilamentar junctions, and a large bloodvessel running between the outer and inner gills at their base, are all well seen (see ante, pp. 130-31).

The ova must be extruded from the generative organ in part by the contraction of the foot compressing the visceral mass. They escape from the generative orifice where the inner lamella of the inner gill is attached anteriorly to the visceral mass, and thence pass on probably as follows. The free portion of the inner gill lamella is converted into a canal by the apposition of the visceral mass to its edge. Behind the foot the ova pass between the united inner lamellae of the inner gills of opposite sides below and the organ of Bojanus above, into the cloaca. This space is small in the Unionacea relatively to their ovaries: it must fill rapidly with ova under pressure, and the shell being closed, there is no other path for them to take but the one which leads into the interlamellar space of the outer gill. This space is open to the cloaca behind the limits marked by the osphradia. Spermatozoa are sometimes found free in the interlamellar spaces of the gills, and as the animals are usually dioecious, it is probable that they are drawn in by the currents of water inhaled by the female, and that the ova are impregnated after their extrusion. The ova may be found in great abundance during the autumn and winter months.

They are nourished by a substance formed by the epithelium of the spongy interlamellar junctions, and develope into the Glochidium. This Glochidium is eventually set free from the parent. It possesses a shell triangular in outline with an incurved tooth in the centre of the free edge of the valve. The margin of each lobe of the embryonal mantle has four sense organs, and a long embryonal byssus filament protrudes from between the valves, by means of which the young animal attaches itself when it quits the egg-membrane. It eventually fixes itself to the fins, tail, etc., of fish, e. g. Stickleback, by means of its valve-teeth. The epidermic cells of its host grow round and enclose it, and it then undergoes a metamorphosis. The permanent mantle is formed, and the rudimentary byssus gland, homologous with the byssus gland, e. g. of Pinna, appears. The embryonal byssus gland of the Glochidium is not homologous with the permanent gland as usually supposed. The nerve ganglia and the otocysts are derived from the ectoderm.

The single adductor of the Glochidium disappears; and the two adductors of the adult are new formations, as is also the shell of the adult.

According to Spengel, the supra-oesophageal ganglia of the Lamellibranch represent the cerebral plus the pleural ganglia, i. e. portions of the parieto-splanchnic ganglia so-called in the Snail, and independent ganglia in Limnaeus and many other Gastropoda. It is a noteworthy point that in Lamellibranchiata there is no connective between the pedal ganglia and the ganglia here called visceral. Supposing that the latter ganglia were the homologues of the pleural and visceral ganglia of e. g. Limnaeus, as ordinarily maintained, the absence of such a connective would be an abnormality. It would not be, on the supposition that the pleural ganglia are fused to the cerebral. The homology of the visceral ganglia with the ganglia of the same name in Gastropoda, is also probable from their connection with an osphradial apparatus.

As to organs of special sense. Certain of the epidermic cells are modified into tactile cells, continuous basally with nerve filaments and furnished at their outer free ends with a bundle of fine projecting tactile hairs or setae. These tactile cells are most plentiful on the papillae of the mantle edge, especially in its siphonal region. They are more sparingly present round the edge of the cloaca, on the edge of the fore part of the mantle, on the labial tentacles, inner surface of mantle, and on the foot.

Eyes are not present in Anodonta at any period of its existence, whether larval or adult. Many larval Lamellibranchiata possess larval eyes at the base of the velum close to the oesophagus. For the eyes of adult Lamellibranchiata, see general account of the Class.

An otocyst or auditory organ lies near to each pedal ganglion. It is surrounded by plasma-cells (p. 115), and consists of a fibrillated connective tissue coat, within which is a nervous layer (?) formed by the auditory nerve, and most internally a layer of ciliated epithelium. Whether special auditory hairs are present, as in Cyclas and some Gastropoda, is uncertain. The cavity of the vesicle contains a fluid in which floats a spherical calcareous otolith, single as in all Lamellibranchiata. The nerve is derived from the cerebro-pedal connective, according to Simroth, not from the pedal ganglion, as ordinarily stated. See Z. W. Z. xxvi. p. 270, Pl. xvii. fig. 56; and for otocyst, Pl. xviii. figs. 62 and 68. It is not certain if the auditory nerve is similarly derived in other Bivalves. It is in other Mollusca. The only specimens of Anodonta at my command were not sufficiently fresh to enable me to decide whether or no the nerve-supply is invariably derived as Simroth describes. It is possible that in some cases the nerve passes through the pedal ganglion, but without being derived from it.

Nervous system.Duvernoy, Memoires de l'Acade'mie des Sciences de lInstitut, xxiv. 1854, p. 87 (with plates referred to). Histology. Vignal, A. Z. Expt. (2) i. 1883. Homologies of, and osphradium. Spengel, Z. W. Z. xxxv. 1881, P. 373

Tactile cells.Flemming, A. M. A. v. 1869; vi. 1870, p. 453; cf. ibid, xxiii.

1883

Otolithic vesicle.Leydig, Lehrbuch der Histologic, 1857, p. 278, with references, p. 283. Simroth, Z. W. Z. xxvi. 1876.

Generative organs in Lamellibranchiata.De Lacaze-Duthiers, A. Sc. N. (4) ii. 1854. Of Mytilus. M'Intosh, A. N. H. (5) xv. 1885. Occasional hermaphroditism in Anodonta. De Lacaze-Duthiers, A. Sc. N (4) iv. 1855.

Passage of ova to gills.Von Baer, Meckel's Archiv. (Archiv. f. Anat. und Physiol.), 1830. Possible passage of ova from one Mussel to another. Von Hessling, Z. W. Z. x. 1859-60, p. 358. The different parts that act as Marsupia. Bronn, Klass. etc. iii. 1, p. 442.

Development with literature.Balfour, Comparative Embryology, i. p. 220. Schierholz, Zur Entwickelungsgeschichte der Teich- und Fluss-muschel, Berlin, 1878; cf. Id. Z. W. Z. xxx. 1878. Post-embryonal development of Najaden (Anodonta). Schmidt, A. N. 51. Cyclas cornea. Ziegler, Z. W. Z. xli. 1885.