Mantle edge is relatively greatly thickened and has sensory organs for 

 touch, and in some species for vision. The muscular mantle edge usually 

 has 3 folds longitudinally. A primary fold may become greatly enlarged and 

 develop several secondary folds. Cells of some folds secrete horny cuticle. 

 Tentacles are always present in definite folds. Over the outer surface of 

 the mantle are many gland cells next to the shell which secrete a sticky 

 substance that becomes impregnated with lime and forms new shell layers. 

 Posterior parts of mantle lobes are modified to form separate openings for 

 inflow and outflow of water. In the branchial chamber hang gills. The 

 upper parts of the gills form the floor of the epibranchial chamber, into 

 which water tubes open. The epibranchial chamber opens into the base of 

 the cloacal siphon, which also receives the end of the rectum from above. 

 The lower or branchial siphon opens into the branchial chamber, but at its 

 base there stretches across its whole upper part a membrane, which covers 

 all but the lower part of the base of the branchial siphon. It does not 

 extend straight across from one side to the other, but presents the 

 appearance of a deep notch extending upward. The mouth lies in the median 

 line between the 2 labial palps and just behind anterior adductor muscle. 

 It is not sharply marked off from the oesophagus. Labial palps take food 

 collected upon the gills from their anterior ends and by cilia pass it into 

 the mouth. The oesophagus proceeds nearly vertically upward to the stomach. 

 The stomach is an enlargement of the alimentary canal, placed in the dorsal 

 part of the visceral mass close to its dorsal wall. The liver mass is 

 closely applied to stomach walls, with ducts that open into the stomach at 

 different points. The stomach narrows posteriorly to open into the 

 convoluted intestine. The crystalline style is a diverticulum of the 

 stomach or intestine. The style is evidently a product of secretion of the 

 epithelium of the diverticulum. Its function is unknown. Liver is paired, 

 one half on each side of visceral mass. The greater part of the visceral 

 mass is made up of the generative mass and the liver. Generative mass is a 

 large gland, surrounding the liver, and forming the posterior part of the 

 visceral mass. In Venus it penetrates into spaces between the uppermost 

 muscle bundles of the foot. The spermatozoan "head" is elongated and 

 conical. The heart lies in an extensive pericardium, which in Venus is a 

 very large cavity. The aortae break up, not into capillaries, but into 

 irregular blood spaces. In the gill filaments blood channels are of more 

 regular size. Blood is colorless and contains many corpuscles. From the 

 ventricle the anterior aorta carries blood forward along the dorsal wall 

 of the visceral mass, over the stomach, then down into the foot. Many 

 branches are given off to liver, sex glands, palps, digestive tract, and 

 foot. In species with a posterior aorta it is distributed to mantle folds, 

 siphons, and posterior adductor. From the irregular sinuses into which 

 arteries empty, blood is collected in larger vessels and carried to the 

 sinus venosus beneath the pericardium. Then it traverses the walls of the 

 nephridia, where waste products are excreted, and on to the gills, then back 

 to the auricles of the heart. Surfaces of the body, particularly those of 

 the mantle, may play a more important part than gills in aerating blood. 

 The nephridium is immediately below the pericardial chamber. Nervous system 

 consists of 3 pairs of ganglia: cerebral, visceral, and pedal. The otocyst 

 is connected with the pedal ganglia. Four gills hang in the branchial 

 chamber, which in Venus is extremely large. Gill filaments have a vascular 

 connection with one another at their inner edges. Spaces are left here and 

 there to allow water to enter the water tube of the gill. The great primary 

 folds include about 7 filaments. These folds are sometimes divided into 2 

 secondary folds. At the primary reentering angle is a partition between 

 lamellae, consisting principally of muscle bundles, between which exists a 

 blood channel. From these channels is sometimes given off a huge blood sinus 

 into the water tube on either side of the partition. A large and active form 

 like Venus must require considerable food and sufficient aeration of blood. 

 The narrow gill filaments, not large enough to contain much blood, seem to be 

 specialized for obtaining food. The large blood sinuses may provide for 

 adequate aeration. The paper is profusely illustrated with drawings. - J.L.M. 



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