The blood of the Anodonta is colourless, and contains colourless corpuscles, which resemble those of Man in structure and present the same Amobiform movements.

The nervous system of the Anadonta consists of three pairs of yellow ganglia; the cephalic, situated at the sides of the mouth; the pedal, placed in the foot; and the parietosplanchnic, on the under face of the posterior adductor muscle. They are united by commissural cords which connect the cephalic ganglia with one another, and with the pedal and parieto-splanchnic ganglia, respectively. The only sense organs which have been discovered, are a pair of auditory vesicles, connected by nervous cords with the pedal ganglia.

The sexes are distinct. The testes and ovaria are similar in character, being racemose glands, which, in the breeding season, occupy a great part of the interior of the body. There is one gland on each side, opening by a minute aperture close to that of the organ of Bojanus.

The spermatozoa have minute, short, rod-like bodies, to which a long, filamentous, active cilium is attached, and, thrown off in enormous numbers, make their way out with the exhalent currents.

The ova are spherical, and the vitelline membrane is produced at one point into a short open spout-like tube, with a terminal aperture, the micropyle, through which, in all probability, the spermatozoon makes its entrance. When fully formed, multitudes of these ova pass out of the oviducal aperture and become lodged in the chambers of the gills, particularly the external gill, which is frequently completely distended by them. Here they are hatched, and give rise to embryos, which are so wholly unlike the parent Anodonta, that they were formerly thought to be parasites, and received the name of Glochidium. The embryo Anodonta is provided with a bivalve shell. Each valve has the form of an equi

lateral triangle united by its base with its fellow, by means of an elastic hinge, which tends to keep the two wide open. The apex of the triangle is sharply incurved, and is produced into a strong serrated tooth, so that when the valves approach, these teeth are directed towards one another. The mantle is very thin, and the inner surface of each of its lobes presents three papillæ, terminated by fine pencils of hairlike filaments. What appears to be the oral aperture is wide, and its margins are richly ciliated. There is a single adductor muscle and a rudimentary foot, from which one or two long structureless filaments, representing the byssus of the sea-mussel, proceed. These byssal filaments become entangled with one another and tend to keep the 'Glochidia' in their places.

After a time the larval Anodonta leave the body of the parent, and attach themselves to floating bodies-very conmonly to the tails of fishes-by digging the incurved points of their valves into the integument in the latter case, and holding on by them as if they were pincers. In this situation they undergo a metamorphosis; the gills are developed, the foot grows, the auditory vesicles become conspicuous in it, and the young Anodonta at length drops off and falls into its ordinary habitation in the mud.

I.

LABORATORY WORK.

In the natural state of the animal only the shell or exoskeleton is visible, or this may be slightly open, and then the edge of the membrane lining it (the mantle) may be visible. Raise one valve of the shell, by separating the mantle from it with the handle of a scalpel, and then cutting through two strong bodies

(the adductor muscles), one at each end of the animal, which run from one valve of the shell to the other and prevent their separation. The two valves will now be united only by their ligament.

2. General form and structure.

a. In the animal now laid bare may be distinguished

A dorsal border turned towards the hinge of the shell, and nearly straight.

A curved ventral border, opposite the dorsal.

A wider anterior end.

A narrower posterior end.

€. A right and left side.

b. The mantle or pallium.

a. A bilobed semitransparent membrane, one lobe lining each valve of the shell.

B. The continuity of the two lobes on the dorsal side of the animal; their separation along most of its ventral side, where each forms a thick yellowish free border.

Y.

8.

The union of the two pallial lobes, for a short distance, towards the posterior part of their ventral border.

The rudimentary dorsal and ventral siphons, separated from one another at the point of union γ and each marked out by a part of the mantle-edge covered by short hair-like processes: the dorsal siphon completely closed below and forming a narrow oval slit; the

C.

ventral siphon open below and continuous with the cleft between the ventral edges of the mantle-lobes.

The branchial or pallial chamber: turn back

the ventral edge of that mantle-lobe from which the shell has been removed: a chamber is thus exposed into which the ventral siphon, and the cleft continuous with it, lead.

ૐ. The cloacal chamber: pass a probe through the dorsal siphon; it will enter a small chamber, separated from the pallial chamber by a partition which unites the hinder part of the two inner gills (c. B).

The contents of the pallial chamber.

α. The foot: a large, yellowish, somewhat ploughshare-shaped mass, in the middle line; its apex directed forwards and ventrally, towards the front of the cleft between the mantle-lobes.

B. The gills or branchiæ: two lamellar bodies on each side of the foot, but reaching farther back than it does: the outer gill on each side, attached to the mantle-lobe; the inner, attached to the foot in front, but farther back, separated by a cleft from it; and behind the foot, united across the middle line with its fellow so as to form a partition separating the cloacal from the pallial chamber.

The labial palps: a pair of small triangular processes on each side, in front of the gills and on the dorsal end of the anterior edge of the foot.

8.

3.

4.

d.

The mouth: each labial palp is continuous with its fellow across the middle line, and between the lip-like ridges thus formed, lies the wide mouth-opening.

The anterior and posterior adductor muscles: if the reflected mantle-lobe be turned down again, the oval divided ends of the adductor muscles can be seen. They appear to perforate the mantle.

Now remove the animal completely from its shell, by detaching the other mantle-lobe from the valve to which it is fixed, and cutting through the attachments of the adductor muscles to that valve. The thick dorsal border of the animal and the continuity of the mantle-lobes will now be more readily made out than they could be previously (2. b. ß).

The heart.

a.

ს.

On the dorsal border of the animal is a clear space, where the mantle is very thin and coversin a cavity filled with fluid. This cavity is the pericardium, and through its walls the heart can be seen beating.

Pin the Anodon out in water between two pieces of loaded cork, or paraffin, so that its dorsal border is upwards, a mantle-lobe spread over each bit of cork, and its foot and gills hanging down between the two pieces: then carefully cut away the dorsal side of the pericardium without injuring the heart.

The heart will now be exposed; it is a yellowish transparent sac, exhibiting regular contrac