9.

Press out an ovary according to its stage of development there will be found in it

α.

B.

7.

Simply ectoderm cells with an unusual preponderance of the smaller form.

Imbedded among cells like a, one which has become larger and clearer than the rest, and possesses a distinct central clear spot in it. Considerable aggregation of granular protoplasm round this cell, so as to form a body consisting of a granular protoplasmic mass, in which is imbedded a clear round vesicle which again contains a distinct rounded dot.

8. The ripe ovum. Consisting of a great irregularly branched mass of protoplasm (vitellus), in which is a clear space (germinal vesicle) containing another body (the germinal spot).

€. The segmented ovum: composed of a large number of small cells. Its thick capsule, rough on its external surface.

XI.

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 all parts of Unio except the shell.

The animal is inclosed 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 Anodon is capable. If some finely divided colouring matter, such as indigo, is dropped into the water, so as to fall towards the

105 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. On the other hand, in a dead Anodonta the valves always gape, and if they are forcibly shut spring open again. The reason of this is the presence of an elastic band, which unites the dorsal margins of the two valves, for some distance, and is put on the stretch when the valves are forcibly brought together. During life they are thus adducted by 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 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; the mouth is median and situated between a projection, which answers to the under surface of the anterior adductor muscle, and the superior attachment of the foot. On each side of the mouth are two triangular flaps with free pointed ends-the labial palpi-and behind these, on each side, two broad, platelike organs, with vertically striated outer surfaces, are visible. These are the gills or branchia. In the dorsal region, the integument is soft and smooth; on each side, it is produced into two large folds, the lobes of the mantle or pallium, which closely adhere to the inner surface of the valves of the shell, and end, ventrally, in the thickened margins already mentioned. They pass into one another in

front of the mouth; at the sides, they are united with the dorsal edges of the outer gill-plates; and, behind, they extend upwards and on to the dorsal face of the body, before finally passing into one another above, and in front of, the anus, which is small, tubular, prominent and median. Thus the anus is inclosed in a part of the cavity bounded by the two mantle lobes, which is relatively small and shallow, and is termed the cloacal chamber; while the gills, the foot, and the palps, hang down into the relatively large branchial chamber, which occupies the space between the mantle-lobes for the rest of their extent. It is the prolongation of the margins of the former cavity which gives rise to the tubular anal siphon seen in so many Lamellibranchs; while the ventral or branchial siphon is a similar prolongation of the margins of the branchial chamber. The dorsal siphon is the channel through which the exhalent currents pass; the ventral, that for the inhalent currents.

The currents are produced and kept up by the action of the cilia which abound upon the gills. The latter are perforated by innumerable small apertures, and the chambers contained between the two lamella of which each gill is formed, are in communication, above, with the cloacal chamber. The cilia work in such a way as to drive the water in which the animal lives from the outer surface of each gill towards its interior. Hence the current which sets from the branchial to the cloacal chamber,

The current of water which is thus continually drawn into the branchial chamber carries with it minute organisms, Infusoria, Diatoms and the like, and many of these are swept to the fore part of the branchial chamber, where they enter the mouth, and are propelled by the cilia which line its cavity into the alimentary canal. The latter presents a short and wide gullet, a stomach surrounded by hepatic follicles, a long intestine coiled upon itself, in a somewhat complicated

manner, and, finally, a rectum, which lies in the middle line of the dorsal aspect of the body, traverses the pericardium and the heart which lies therein, and finally ends in the anus. As the mouth is below and behind the anterior adductor and the rectum passes in front of and above the posterior adductor, it is clear that the alimentary canal, as a whole, lies between the two adductor muscles.

Digestion, that is solution of the proteinaceous and other nutritive matters contained in food, is effected in the stomach and intestine; and the nutritious fluid, thus formed, transudes through the walls of the alimentary cavity and passes into the blood contained in the blood-vessels which surround it. This blood is thence carried into a large sinus, which occupies the middle line of the body under the pericardium and between the organs of Bojanus (see Laboratory work 5), and receives the greater part of the blood returning from all parts of the body. From this median vena cava, branches are given off to the gills and open into the extensive vascular network which those organs contain. From this, again, trunks lead towards the pericardium and open into one or other of the two auricles of the heart, which communicate by valvular apertures with the ventricle. The ventricle gives off two aortic trunks, one of which, the anterior, runs forwards in the middle line, above the rectum, while the other runs backwards, below the rectum. From these two aortæ branches are given off which divide into smaller ramifications for the different regions of the body, and for the viscera, and finally terminate in channels which answer to the capillaries of the higher animals.

The pericardial cavity, in which the heart is lodged, is situated in the posterior half of the dorsal region of the body. Through its thin dorsal wall, and, still better, when it is carefully laid open, the heart can be seen beating. The auricles contract and, after them, the ventricle; the wave-like contrac