a.

Immature spermatozoa are at times present in abundance. They can be recognised by the relatively large size of the 'head', which is oval and can be distinctly seen to lodge a small nucleus. (Cf. Lumbricus and Rana.)

b. The ripe ovum. Examine under a high power.
Its shape; usually round, more rarely oval.
B. The absence of a distinct vitelline membrane.
The protoplasm; consisting of a central uniformly
granular vitellus, which graduates peripherally
into a superficial clear layer.

Y.

8. The germinal vesicle; a large round structure, usually containing one immense germinal spot. L. Development. (The Pond Snail, Lymnæus stagnalis.) See p. 278.

Examine the developing eggs from time to time, and look especially for

a. The fertilized ovum (oosperm) during segmentation.
It divides up into two sets of cells-a smaller
rapidly dividing clearer set, and a larger slowly
dividing yolk-laden set (the presence of the yolk
gives these an opaque coarsely granular appear-
ance). Look out for polar bodies (see p. 278).
b. The same at the close of segmentation. Looked at
from the outside the larger cells appear as a dark
mass, lying within the smaller more transparent
ones, which have now completely overgrown them.
C. The gastrula phase, immediately following upon b.
Examine from beneath and note

a.

the blastopore; a small slit-like orifice on the flattened under surface.

Examine from the side in optical section.

B. The archenteron; a sac-like pit opening externally by the blastopore, its walls are formed of the invaginate yolk-bearing endoderm cells.

7. The ectoderm; a single layer of investing cells, the product of the more transparent rapidly dividing ones referred to above.

The cleavage-cavity or blastocale; a spacious cavity, enclosed between the investing and invaginate layers.

d. The Trochosphere larva; recognizable by its eggshaped contour and rotatory movements.

a. The mouth; a small median orifice situated at the enlarged end.

B. The trochal ridge; a saddle-like band encircling the dorso-lateral area pre-orally. cilia.

Examine in optical section and note

Look for its

γ. The foot (it first appears at this stage); a median ventral outgrowth of the body wall just behind

the mouth.

8. The stomodaum; a blind sac-like involution of the integument, its aperture giving rise to the mouth.

E.

The archenteron; now partially surrounded by a conspicuous large-celled granular mass - the digestive gland, arising as an outgrowth of its

wall.

Note the bilateral symmetry of the larva at this period. The blastopore appears shifted back, as the result of elongation of the embryo and

e.

displacement, upon the development of the foot. An integumental pit known as the shell-gland, appears at the hind end of the body during this stage.

The Veliger larva. A head-segment is now becoming differentiated, and the rotatory movements of the animal are less marked. Bilateral symmetry is becoming disturbed. Note especially

The velum; a ciliated pre-oral fold, occupying the position of the original trochal-ridge.

B. The foot; now greatly increased in size and utilized for purposes of locomotion.

Y.

The eye-spots; two black masses at the sides of the head segment.

f. The advanced larva. Tentacles are now appearing, as paired outgrowths of the cephalic wall (the eyes are not carried up by them as in Helix). The original bilateral symmetry no longer obtains. Note the beat of the heart; the presence of the visceral sac and of its related shell-at this stage a transparent chitinous cap. The velum is undergoing a marked reduction.

If any difficulty is experienced in examining the embryos in the living state, they should be liberated from the surrounding albuminous investment by means of a couple of needles, and submitted, for 20 minutes, to the action ofp.c. Osmic acid solution. They may then be transferred to alcohol of increasing strength, and preserved for future examination.

V.

THE FRESH-WATER MUSSEL

(Anodonta Cygnæa).

UNDER the name of 'Fresh-water Mussel' two distinct kinds of animals, which are not unfrequently abundant in our ponds and rivers, are included; namely, the Anodonta and two or three kinds of Unio. The Anodonta is chosen for special study here, but what is said about it applies very well to nearly all parts of Unio except the shell.

The animal is enclosed in a shell composed of two pieces or valves, which are lateral, or right and left, in relation to the median plane of the body. The more rounded and broader end is anterior, the more tapering, posterior. If placed in a vessel of water, at the bottom of which there is a tolerably thick layer of soft mud or sand, and left quite undisturbed, the Anodonta will partially bury itself with its anterior end directed obliquely downwards; and the valves will separate at their ventral edges for a short distance. At the edges of this 'gape' of the shell the thickened margins of a part of the contained body which is called the mantle, become visible, and between them a large, whitish, fleshy, tongue-shaped structure-the foot-not unfrequently protrudes, and is used to perform the sluggish movements of which the Anodonta is capable. Mud 'tracks' are left behind the animal when in motion, as can be seen on observing its natural habit in slow running or still shallow

waters.

If some finely dividing colouring matter, such as indigo, is dropped into the water, so as to fall towards the gape, it will be seen to be sucked in; while, after a short time, a current of the same substance will flow out from an opening between the two edges of the mantle on the dorsal side of the posterior end of the body; and these 'inhalent' and 'exhalent' currents go on, so long as the animal is alive and the valves are open. Any disturbance, however, causes the foot, if it was previously protruded, to be retracted, while the edges of the mantle are drawn in and the two valves shut with great force. This adduction results from the contraction of two thick bundles of muscular fibres, which pass from the inner face of one valve to that of the other, one at the anterior and the other at the posterior end of the body, and are called the anterior and posterior adductors. The valves of the dead Anodonta always gape, and if they are forcibly shut they spring open again. The reason of this is the presence of an elastic band or ligament, which unites the dorsal margins of the two valves, for some distance, and is put upon the stretch when the valves are forcibly brought together. The natural tendency to divarication of the valves resulting from this, is held in check during life by the contractions of the aforenamed adductor muscles. These respond to a nervous stimulus, and, on the death of the animal, that being withdrawn, the full force of the elasticity comes into play, the valves becoming divaricated to their utmost extent.

The animal can be extracted from the shell without damage, only by cutting through these muscles close to their attachments. It is bilaterally symmetrical, the foot proceeding from the middle of its ventral surface. There is a total absence of any distinct head segment such as is seen in the Snail. The mouth is median and situate between