Elementary Zoology, Second Edition

by Vernon L. Kellogg

Before the removal of the valve, as just described, notice a portion of the mantle adhering to the inner face of the valve, along a line of attachment indicated by a crease. This is the pallial line. After the left valve has been removed, the mantle being carefully separated from it, note the large conical projections from the valves, the hinge teeth, which fit into each other. Note the large muscle impression just in front of the hinge-teeth; this is the point of attachment of the anterior adductor muscle, while just behind and adjoining it is the impression of the anterior retractor muscle. Note posterior to the adductor and below the retractor a small impression which affords attachment for the protractor muscles of the foot. At the other end of the valve, note the large impression of the posterior adductor muscle with the impression of the small posterior retractor muscle just above it.

Technical Note.—Lift back the left mantle-lobe, thus exposing the body parts underneath.

Note the projecting muscular foot, the movements of which are governed by the retractor and protractor muscles attached to the impressions just mentioned. Note a pair of flattened plate-like structures composed of thin, ribbed, membranous folds. These are the gills. Note just beneath the anterior adductor muscle a small opening leading into the soft visceral mass of the body. This is the mouth. Note near the mouth two pairs of plate-like structures much smaller than the gills. These are the labial palpi, and it is by their action that food-particles which have been brought in with the water are conveyed to the mouth. Note at the posterior part of each mantle-lobe a fringed portion which, together with a corresponding part on the other side, forms the inhalant siphon. The cilia of the fringes carry water and food-particles into the space enclosed by the mantle-lobes; this space is the mantle-cavity. After the food has been taken out and the water has passed through the finely striated gills it is collected in a common cavity which extends above the two sets of gills on each side. This space is called the supra-branchial cavity. This cavity is continuous posteriorly with a space between the right and left mantle-lobes, which is connected with the exterior by an opening above the inhalant siphon called the exhalant siphon. The function of the gills is partly to produce currents of water carrying the food to the mouth, and partly respiratory. The mantle is an important organ of respiration.

Make a drawing showing the organs described.

Technical Note.—Carefully cut away the mantle and gills from the left side, and also the labial palpi, being careful not to disturb the visceral mass.

Note two openings along the line where the gills and foot come together. The uppermost is the opening of the ureter giving exit to the excretion from the kidneys; the lower is the opening of the duct from the reproductive organs and is called the genital aperture. The products from both of these organs are carried out through the exhalant siphon.

Note that the mouth leads by a short tube (œsophagus or gullet) into a large cavity, the stomach, which is surrounded by a greenish mass, the digestive gland.

Technical Note.—Carefully cut the delicate covering of the dorsal portion of the visceral mass and expose a cavity.

The cavity thus exposed is the pericardium. Note within the pericardium a long tube extending through it. This is a portion of the alimentary canal, the rectum, which opens posteriorly through the anus into the supra-branchial chamber. Note a muscular sac about the rectum midway of its course through the pericardium. This is the unpaired ventricle of the heart. Attached to each side of the ventricle are thin-walled sacs, the right and left auricles, which are entered by fine blood-vessels, the efferent branchial veins, from the right and left gills. The blood brought through these blood-vessels from the gills flows into the auricles and from them into the unpaired muscular ventricle, from which it is forced anteriorly and posteriorly through two main arteries, the anterior and posterior aortas, to all parts of the body. After bathing the body-tissues the blood is collected into a median longitudinal vein beneath the pericardium called the vena cava. From the vena cava the blood passes through the kidneys and gills to be returned at last to the heart. The mantle acts as an organ for the aeration of the blood, and the blood it receives or at least part of it passes directly back to the heart without passing through the kidneys and gills.