Page:EB1911 - Volume 16.djvu/139

It does not contain blood or communicate directly with the blood-system; this isolation of the pericardium we have noted already in Gastropods and Cephalopods. A good case for the examination of the question as to whether blood enters the pericardium of Lamellibranchs, or escapes from the foot, or by the renal organs when the animal suddenly contracts, is furnished by the Ceratisolen legumen, which has red blood-corpuscles. According to observations made by Penrose on an uninjured Ceratisolen legumen, no red corpuscles are to be seen in the pericardial space, although the heart is filled with them, and no such corpuscles are ever discharged by the animal when it is irritated.

Fig. 19.—Nerve-ganglia and Cords of three Lamellibranchs. (From Gegenbaur.)
A,	Of Teredo.
B,	Of Anodonta.
C,	Of Pecten.
a,	Cerebral ganglion-pair ( = cerebro-pleuro-visceral).
b,	Pedal ganglion-pair.
c,	Olfactory (osphradial) ganglion-pair.

The pair of renal organs of Anodonta, called in Lamellibranchs the organs of Bojanus, lie below the membranous floor of the pericardium, and open into it by two well-marked apertures (e and f in fig. 18). Each nephridium, after being bent upon itself as shown in fig. 18, C, D, opens to the exterior by a pore placed at the point marked x in fig. 1 (5) (6). One half of each nephridium is of a dark-green colour and glandular (h in fig. 18). This opens into the reflected portion which overlies it as shown in the diagram fig. 18, D, i; the latter has non-glandular walls, and opens by the pore k to the exterior. The renal organs may be more ramified in other Lamellibranchs than they are in Anodonta. In some they are difficult to discover. That of the common oyster was described by Hoek. Each nephridium in the oyster is a pyriform sac, which communicates by a narrow canal with the urino-genital groove placed to the front of the great adductor muscle; by a second narrow canal it communicates with the pericardium. From all parts of the pyriform sac narrow stalk-like tubes are given off, ending in abundant widely-spread branching glandular caeca, which form the essential renal secreting apparatus. The genital duct opens by a pore into the urino-genital groove of the oyster (the same arrangement being repeated on each side of the body) close to but distinct from the aperture of the nephridial canal. Hence, except for the formation of a urino-genital groove, the apertures are placed as they are in Anodonta. Previously to Hoek’s discovery a brown-coloured investment of the auricles of the heart of the oyster had been supposed to represent the nephridia in a rudimentary state. This investment, which occurs also in many Filibranchia, forms the pericardial glands, comparable to the pericardial accessory glandular growths of Cephalopoda. In Unionidae and several other forms the pericardial glands are extended into diverticula of the pericardium which penetrate the mantle and constitute the organ of Heber. The glands secrete hippuric acid which passes from the pericardium into the renal organs.

Fig. 20.—Otocyst of Cyclas. (From Gegenbaur.)
c,	Capsule.
e,	Ciliated cells lining the same.
o,	Otolith.

Nervous System and Sense-Organs.—In Anodonta there are three well-developed pairs of nerve ganglia (fig. 19, B, and fig. 1 (6)). An anterior pair, lying one on each side of the mouth (fig. 19, B, a) and connected in front of it by a commissure, are the representatives of the cerebral and pleural ganglia of the typical Mollusc, which are not here differentiated as they are in Gastropods. A pair placed close together in the foot (fig. 19, B, b, and fig. 1 (6), ax) are the typical pedal ganglia; they are joined to the cerebro-pleural ganglia by connectives.

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-pedal and pleuro-pedal connectives, however, in these cases are only separate in the initial parts of their course, and unite together for the 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 union of a visceral pair with the cerebro-pleural.

Fig. 21.—Pallial Eye of Spondylus. (From Hickson.)
a,	Prae-corneal epithelium.		f,	Retinal nerve.
b,	Cellular lens.		g,	Complementary nerve.
c,	Retinal body.		h,	Epithelial cells filled with pigment.
d,	Tapetum.		k,	Tentacle.
e,	Pigment.

The sense-organs of Anodonta other than the osphradia consist of a pair of otocysts attached to the pedal ganglia (fig. 1 (6), ay). The otocysts of Cyclas are peculiarly favourable for study on account of the transparency of the small foot in which they lie, and may be taken as typical of those of Lamellibranchs generally. The structure of one is exhibited in fig. 20. 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 majority of the Protobranchia the orifice of invagination remains open throughout life; this is also the case in Mytilus including the common mussel.

Fig. 22.—Two Stages in the Development of Anodonta. (From Balfour.) Both figures represent the glochidium stage.
A,	When free swimming, shows the two dentigerous valves widely open.
B,	A later stage, after fixture to the fin of a fish.
sh,	Shell.
ad,	Adductor muscle.
s,	Teeth of the shell.
by,	Byssus.
a.ad,	Anterior adductor.
p.ad,	Posterior adductor.
mt,	Mantle-flap.
f,	Foot.
br,	Branchial filaments.
au.v,	Otocyst.
al,	Alimentary canal.

Anodonta has no eyes of any sort, and the tentacles on the mantle edge are limited to its posterior border. This deficiency is very usual in the class; at the same time, many Lamellibranchs have tentacles on the edge of the mantle supplied by a pair of large well-developed nerves, which are given off from the cerebro-pleural ganglion-pair, and very frequently some of these tentacles have undergone a special metamorphosis converting them into highly-organized eyes. Such eyes on the mantle-edge are found in Pecten, Spondylus, Lima, Pinna, Pectunculus, Modiola, Cardium, Tellina, Mactra, Venus, Solen, Pholas and Galeomma. They are totally distinct from the cephalic eyes of typical Mollusca, and have a different structure and historical development. They have originated not as pits but as tentacles. They agree with the dorsal eyes of Oncidium (Pulmonata) in the curious fact that the optic nerve penetrates the capsule of the eye and passes in front of the retinal body (fig. 21), so that its fibres join the anterior faces of the nerve-end cells as in Vertebrates, instead of their posterior faces as in the cephalic eyes of Mollusca and Arthropoda; moreover, the lens is not a cuticular product but a cellular structure, which, again, is a feature of agreement with the Vertebrate