be seen from the surface. In sexually mature and 

 "fat" oysters they are obscured by the deposits 

 of glycogen and by the accumulation of sex cells. 

 The paired lateral afferent veins (l.af.v.) arc of 

 smaller diameter than their ct>mmon partner. 

 They are located along tlie axis of tlie outer 

 ascending lamella where the lamella fuses with 

 the mantle lobe. The lateral afferent veins 

 receive the blood fr(im the mantle through the 

 pallial veins (p. v.). 



At regular intervals the common afferent vein 

 is connected with the lateral veins by short trans- 

 verse (horizontal) vessels (t.v.). These vessels 

 can be seen in injected preparations of tlie gills 

 and in sectioned material. The communication 

 between the horizontal vessels in the gill tissues 

 is maintained bj* means of vertical vessels which 

 emerge from the walls of the three afferent veins 

 as a series in a double row, one following tlie inner 

 and the other the outer lamella of the demibranch. 

 At each interfilamentar shelf the vertical vessels 

 empty into a lacuna and eventually into the tubes 

 of the gill filaments. There is no special path for 

 the return of the blood from the interfilamentar 

 lamellae and the tubes because the filaments end 

 blindly. The walls of the common afferent vein 

 contain a layer of elastic fibers arranged circularly; 

 they are scarce in the walls of other veins. Endo- 

 thelium is absent in all these vessels. The walls 

 of the vertical vessels of the lamellae have a layer 

 of muscular fibers which are able at intervals to 

 constrict the lumen of the vessels along their 

 length. In this way the flow of blood inside tlie 

 gills is regulated (Elsey, 1935). 



The blood channels in the interlamellar junctions 

 are in communication with the vertical vessels and 

 provide for the passage of blood from one lamella 

 to the other. This rather inefficient circulation 

 of the blood in the gill vessels is influenced by the 

 contraction of the entire gill musculature and by 

 contractions of the major afferent and efferent 

 veins. The pulsations of these vessels have not 



been observed in vivo, but their histological 

 structure suggests that they are capable of con- 

 stricting their lumen. A tangential section of the 

 common afferent vein preserved in a relaxed state 

 (fig. 2.34) shows a well-developed layer of circular 

 muscles flanked on both sides by thin bands of 

 longitudinal muscles. 



The system of efferent vessels comprises two 

 short common efferent veins (c.ef.v.) which open 

 into the auricles, a pair of branchial efferent 

 veins (br.ef.v.) which run along the axis of the gill 

 lamellae (fig. 73), pallial efferent veins (not shown 

 in fig. 233), and the interlamellar and interfila- 

 niental vessels (il.v.) of the gills. The branchial 

 efferent veins (br.ef.v.) run along the giU axis 

 parallel to the branchial nerves (fig. 73) at the 

 junctions of the ascending and descending lamellae. 

 In their course they receive blood from the renal 

 sinuses and empty into the common efferent vein. 

 Blood which circulates in the mantle is carried 

 to the heart through pallial sinuses and veins, 

 but part of the blood from the posterior portion 

 is drained back to the gills and to the branchial 

 efferent vein (br.ef.v.). 



^'■'^riry 



Figure 234. — Photomicrograph of a tangential section of 

 the wall of the common afferent vein of C. virginica 

 preserved in fully relaxed state. Narcotized in mag- 

 nesium sulfate. Kahle, hematoxylin-eosin. 



was drawn from a number of preparations of partially injected venous sj'stem. Only the approximate position of 

 various vessels is indicated. The diagram does not intend to show the actual appearance and distribution of veins. 

 A — auricle; V — ventricle; a. — anus; ad. m. — adductor muscle; br.ef.v. — branchial efferent vein; c. af. v. — common 

 afferent vein; c. v. — cephalic vein; c. ef. v. — common efferent vein; g. — gills; g. v. — gastric veins; h. v. — hepatic 

 veins; il. v. — interlamellar veins of the gills; 1. af. v. — lateral afferent vein; 1. p. — labial palps; 1. v. — labial vein; 

 m — mantle; m. s. — mantle sinus; p. v. — pallial vein ; py. c. — pyloric caecum; p. d. v. — posterior dorsal vein; p. v. v. — 

 posterio-ventral vein; t. v. — transverse veins of the gills; r. — rectum; r. s. — renal sinus; r. g. v. — reno-gonadial veins; 

 r. v. — renal vein; v. s. — visceral mass; w. t. — water tubes of the gills. 



CIRCULATORY SYSTEM AND BLOOD 



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