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.