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of the anterior adductor and below that of the anterior retractor. Its fibres spread out in a fan-like manner over the upper part of the foot, some of them extending over the surface of the liver.

The lesser retractors. Several very small muscles arising from the shell just in front of the umbo and spreading over the surface of the liver.

. The intrinsic foot-muscles: forming the greater part of the ventral portion of that organ.

Small muscles attached to each mantle-lobe, at some little distance from its swollen free edge and fixed to the shell along a linear impression, which runs from one adductor to the other and is termed the pallial line.

Tease out in glycerine a bit of one of the muscles which has been treated with 0.5% chromic acid solution. Examine with inch obj. It

is composed of spindle-shaped flattened cells, in each of which lies an elongated nucleus: the substance surrounding the nucleus is clear, but the rest of the cell is granular and contains a great number of small particles arranged pretty definitely in transverse rows. While these muscular fibres agree in form with those of smooth muscles, in minute structure they approach striped muscles.

12. The shell or exoskeleton.

a.

Its two hardened lateral pieces or valves; each with a straight dorsal and a curved ventral edge,

a.

and an anterior larger and posterior smaller end note the soft uncalcified ventral edge of each valve.

The umbo; a small blunt eminence on the dorsal border of each valve near its anterior end.

The ligament: an elastic uncalcified part of the exoskeleton behind the umbones, uniting the two valves and tending to keep their ventral edges slightly separated.

The markings on the shell.

External markings. The outside of the shell is greenish brown, and on it are seen a number of concentric lines generally parallel to the margin of the shell, and more numerous towards the ventral edge.

B. Internal markings. The interior of the valve is white and iridescent: on it are seen, near the dorsal border, two oval marks, the anterior and posterior adductor impressions.

Joining the two adductor impressions is a curved line, the pallial impression, which marks where the muscles of the edge of the mantle were fixed to the shell.

In front of the posterior adductor impression is seen a small mark, indicating where the posterior retractor muscle was fixed.

Behind the anterior adductor impression are two marks, one opposite its upper, the other opposite its lower end: the former indicates the point of attachment of the

anterior retractor, the latter of the protractor pedis muscle.

Extending from each adductor impression towards the umbo is a fainter, gradually tapering impression, which may be followed into the cavity of the umbo, and indicates the successive attachments of the adductor muscles, as the animal has increased in size.

13. In the breeding season, examine the contents of the testis for spermatozoa, and those of the ovary for

ova.

Note the micropyle of the latter. If the outer gill appear to be thick and distended, it will be found full of the larvæ of the Anodon,-Glochidium. Note the characters of their shells and the entangled filaments, or byssus, with which they are provided.

XII.

THE FRESH-WATER CRAYFISH (Astacus fluviatilis) AND THE LOBSTER (Homarus vulgaris).

THE Crayfish and the Lobster are inhabitants of the water, the former occurring in many of our rivers and the latter abounding on the rocky parts of the coasts of the European seas. They are bilaterally symmetrical animals, provided with many pairs of limbs, among which the large prehensile 'claws' are conspicuous. They are very active, walking and swimming with equal ease and sometimes propelling themselves backwards or forwards, with great swiftness, by strokes of the broad fin which terminates the body. They have conspicuous eyes, mounted upon moveable stalks, at the anterior end of the head; and two pairs of feelers, one pair of which are as long as the body, while the other pair are much shorter.

The body and limbs are invested by a strong jointed shell, or exoskeleton, which is a product of the subjacent epidermis, and consists of layers of membrane which remain soft and flexible in the interspaces between the segments of the body and limbs, but are rendered hard and dense elsewhere by the deposit of calcareous salts; the exoskeleton is deeply tinged with a colouring matter which turns red when exposed to the action of boiling water. The body presents an anterior division-the cephalothorax-covered

by a large continuous shield, or carapace; and a posterior division-the abdomen-divided into a series of segments which are moveable upon one another in the direction of the vertical median plane, so that the abdomen can be straightened out, in extension; or bent into a sharp curve, in flexion. Of these segments there are seven. The anterior six are the somites of the abdomen, and each of them has a pair of appendages attached to its ventral wall. The seventh bears no appendages and is termed the telson. The anus is situated on the ventral aspect, beneath the telson and behind the last somite.

A groove on the surface of the carapace, which is termed the cervical suture, separates an anterior division, which belongs to the head or cephalon, from a posterior division which covers the thorax; and the thoracic division of the carapace further presents a central region, which covers the head, and wide lateral prolongations, which pass downwards and cover the sides of the thorax, their free ventral edges being applied against the bases of the thoracic limbs. These are the branchiostegites. Each roofs over a wide chamber in which the gills are contained and which communicates with the exterior, below and behind, by the narrow interval between the edge of the branchiostegite and the limbs. Anteriorly and inferiorly, the branchial chamber is prolonged into a canal, which opens in front and below at the junction of the head with the thorax. In this canal there lies a flat oval plate-the scaphognathite-which is attached to the second pair of maxillæ and which plays a very important part in the performance of the function of respiration. Of the thoracic limbs themselves there are eight pairs, and, on the ventral face of the body, the lines of demarcation between the eight somites to which these limbs belong may be observed. There is no trace of any