FIG. 1. The common Oyster (Ostrea edulis), dissected so as to show the principal features of its anatomy.

Lamellibranchiata.

Plate VII. Lamellibranchiata.

The animal has been removed from its shell and dissected on its left side, the one that corresponds to the flat or free valve of the shell. It may therefore be compared without difficulty with the figure of Anodonta, Plate vi., which has been dissected in the same way.

a. a. a. Right lobe of the mantle which has been left entire, the left lobe being cut away save at its oral end.

a. Anterior dorsal angle.

a". Anterior ventral angle, which is produced a little beyond the dorsal angle. The part included between these two angles is fitted into a deep recess in the right valve, and the ligament of the shell, which is internal, corresponds to the straight edge uniting them.

b. The two oral tentacles of the left side. The tentacles are not quite symmetrical in this animal. Note the deep bay which lies between them and the cut edge of the left mantle which is remarkably thickened in this region.

c. The gills or branchiae. There are four of these as in Anodonta. They are not symmetrical inter se, and are fluted, i.e. the lamellae are not flat but undulated.

c'. The spot where the attachment of the gills to the mantle lobes ends.

This attachment divides the mantle cavity into an inhalent, oral or infra-branchial chamber, of great length but shallow in the Oyster, and an exhalent, aboral or supra-branchial chamber, which is deep and of less extent than the oral chamber. Note the four lines of apertures into the interlamellar spaces of the gills.

d. The single adductor muscle, the sole adductor present in the families Ostraeidae and Aviculidae. The Lamellibranchiata have been divided into the Monomyaria with one, and Dimyaria with two adductor muscles, as in Anodonta, but the division is not a good one. This single adductor corresponds to the posterior adductor of Anodonta. It is distinctly divisible into two parts, an opaque portion close to the pericardium, and a translucid portion behind. This division is observable in the adductors of many Lamellibranchiata. both anterior and posterior. Coutance has stated that the muscular fibres of the translucid portion in Pecten are striated, of the opaque, smooth; that the former contract rapidly, the latter slowly; that the opaque portion is more like a ligament in function. But the histological difference between the parts does not extend to other Lamellibranchiata.

e. The left auricle, and e'. The single ventricle of the heart. The auricles are fused together in the middle, while their two extremities are free, i. e. they receive blood from the gills by two channels and communicate with the ventricle by two short vessels, the ventricular orifices of which are guarded by two valves apiece. The ventricle is not perforated by the intestine, a point in which the Oyster resembles Anomia and Teredo. The heart lies in a pericardium situated anteriorly to the adductor and communicating with the exterior through the nephridium.

f. The body of the animal which contains the digestive tract, the liver or hepato-pancreas, the organs of generation, and a part of the renal organ. A short 'oral process' of Hoek, which contains a loop of intestine (Fig. 2, c.), projects from it just in front of the visceral ganglion, g.

g. The left visceral ganglion, of very large size, and when seen from the surface instead of sideways, bilaterally symmetrical. It gives off posteriorly and laterally nerves which branch repeatedly, possess a muscular sheath (Hoek), and are connected at the margin with the pallial nerve. This nerve is complete, i. e. extends along the whole edge of the mantle. Anteriorly the ganglion gives off in addition to the cerebro-pleural commissure two nerves, one of which passes between the adductor and the pericardium to the mantle. The other is the branchial nerve, and is seen passing along the commissure of the two left gills. Between this nerve and the cerebro-pleural commissure, and behind the spot where they cross one another, is a depression, the common vestibule of the renal and generative ducts.

h. Cerebro-pleural ganglion of the left side sending forwards branches to the pallial nerve. There is no foot and in consequence no pair of pedal ganglia. These ganglia are either fused with the cerebro-pleural or else aborted; but there is a nerve-cord passing beneath the mouth from one to the other cerebro-pleural ganglion which represents the cord connecting the two pedal ganglia of Anomia. In this animal they are approximated to the cerebro-pleural ganglia and are of large size.

i. Commissure between the cerebro-pleural and visceral ganglia. Two nerves going to the body originate from it.

j. Large funnel-shaped anus.

The organs of generation as described by Hoek extend over the surface of the body and the anterior surface of the pericardium. There are two glands, a right and left, but they are connected peripherally. They form a system of anastomosing and interlacing channels beneath the integument from which caeca descend vertically into the body. In these caeca the generative products both male and female develope side by side. Each duct opens anteriorly into a recess common to it and the duct of the organ of Bojanus or nephridium of the same side.

The nephridium consists of long caeca which give origin to other caeca and extend even into the mantle. They lie in the body superficially to the genitalia. The renal channels open into a long chamber. This chamber or duct communicates with the recess above mentioned and by a reno-pericardiac canal with the pericardium. The nephridium of the Oyster differs strikingly from that of other Lamellibranchiata (Fig. 4) in not being compact. Some parts of the renal channels are covered with a ciliated, other parts with a stratified epithelium.

Hoek, Tijdschrift der Nederlandsche Dierkundige Vereeniging, Suppl. i. Leiden, 1883, (in French and Dutch).

A resume of anatomy of the renal and genital organs. Abstract in Journal of Roy. Micr. Soc. (2), iii. 1883, pt. 1, p. 354.

Nervous system. Duvernoy, Memoires de l'Institut, xxiv. 1854, Monograph, p. 61.

Visual organs of O. Virginica. Sharp, Mitth. Zool. Stat. Naples, v. 1884.

Adductor muscle. Plateau, A. Z. Expt. (2), ii. 1884.

Sexes of oysters. Ryder, A. N. H. (5), xii. 1883. O. Virginica and O. angu-lata have the sexes separate; Bouchon-Brandely, C. R. xcv. 1882.

Development. Horst, A. N. H. (5), ix. 1882 .; Brooks, Johns Hopkins Univ. Biological Studies, Report, Chesapeake Zool. Lab., Session 1878; Ryder, Report of U. S. Commission of Fish and Fisheries for 1882 (published 1884).

Green colour in oysters. Puysegur, cf. Journal Roy. Micr. Soc. iii. 1880; Ryder, ibid. (2), iii. 1883; both in full in Reports of U. S. Commission of Fish and Fisheries for 1882 (published 1884). Ray Lankester, Q. J. M. xxvi. 1886.

FIG. 2. Digestive tract of the same, in the position it would occupy in a specimen placed like the one figured above (Fig. 1). After Brandt and Ratzeburg, Medizin. Zoologie, Berlin, ii. 1833, PI. XXXVI. Fig. 4.

a. The two pairs of labial tentacles.

b. Anal termination of the intestine, lying to the dorsal side of the adductor muscle, j. Fig. 1. The anus is funnel-shaped.

c. Loop of intestine occupying the 'oral process' of Hoek, which projects in front of the visceral ganglia (g, Fig. 1).

d. Stomach, upon the side of which, turned towards the dissector, lies the coil of intestine immediately preceding the anus.

FIG. 3. Heart and principal vessels of Arca Noe. From Poli, Testacea utriusque Siciliae, ii, Parma, 1795, P1. XXV. Fig. 2.

The heart is aberrant, inasmuch as there are not only two auricles as in other Lamellibranchiata, but the ventricle also is double.

a. Branchial venous, i. e. efferent vessels. The shaded vessel below represents the branchial arterial, i. e. afferent vessels.

b. Mantle vessels, entering with a.

c. The auricle of the left side.

d. d. The two ventricles, left and right, the former shaded. Each ventricle gives off e. A single trunk which divides at once into two vessels, an anterior and posterior.

h. Anterior aorta formed by the fusion of the two anterior vessels from the left and right sides respectively. It passes forwards and supplies the viscera, labial tentacles and anterior part of the mantle.

g. Posterior aorta formed by the fusion of i. The two posterior vessels.

j.j. The organ of Bojanus or nephridium.

FIG. 4. Longitudinal section, partly diagrammatic, of the left organ of Bojanus, or nephridium of Unio pictorum. From de Lacaze-Duthiers, A. Sc. N. (4), iv. 1855, PI. V. Fig. 2.

a. Posterior adductor muscle.

b. Ventricle of heart traversed by the intestine, and giving off the anterior and posterior aortae. b'. br. Pericardial space, immediately beneath the dorsal surface of the animal.

c. Rectal termination of intestine with anus.

d. Dorsal edge of mantle in section.

e. External aperture of nephridium. It leads into the duct, a thin-walled sac lying immediately below, i. e. ventral to, the pericardial space. The two ducts, left and right, sometimes communicate beneath the pericardial space anteriorly, e. g. in Anodonta.

f. Internal aperture of the nephridium into the anterior extremity of the pericardial space. The arrow shows that the anterior part of the glandular or lamellate portion of the organ passes internally to the duct.

g. Aperture of the generative organ, lying close to but separate from the nephridial duct in this mussel, and in the majority of Lamelli-branchiata. h. The aperture from the non-glandular into the glandular portion of the nephridium. The former has smooth walls, the latter walls produced into lamellae. The cells covering these lamellae are pigmented and contain phosphatic concretions.

It is clear that water set in motion by the ciliated epithelium of the organ can either enter or pass out of the pericardial space.

FIG. 5. Veliger of Cardium pygmaeum before it quits the egg. From Loven, copied in Bronn, Klassungen u. Ordnungen des Thierreichs, vol. iii. (1), 1862, P1. XXXVIII. Fig. 31.

a. Entire circular velum, fringed with cilia, and bearing in its centre a long flagellum. The velum, according to Loven, is formed by the coalescence of two papillae. It is never produced into lobes, as in Gastropoda and Pteropoda. In some fresh-water forms which have no free larva the velum is reduced (Anodonta, Unto, Cyclas), or aborted (Pisidium). In these forms, in Teredo and Ostraea edulis, the central flagellum is absent.

b. Archenteron inclosed by hypoblast cells.

c. Body cavity (?).

d. d. Epiblast. The cells forming this layer divide rapidly and surround the large, slowly dividing hypoblast cells, thus forming a Gastrula by overgrowth or epibole.

In an early stage the embryo is covered with short cilia. In a later stage than the one figured the two shell-valves appear, at first meeting without a hinge. They cover the velum which becomes retractile. The mantle-folds grow. The archenteron acquires hepatic diverticula; a stomodaeum and proctodaeum are formed; and the foot grows out between mouth and anus. The anterior appears before the posterior adductor. At this stage the embryo is hatched.

Cf. Loven, Archiv fur Naturgeschichte, 15, 1, 1849, p. 317; Balfour, Comparative Embryology, vol. i. 1880, pp. 214, 216-17.