being discharged through the uriiKigenital vesti- 

 bule. On each side of the auricular part of the 

 pericardium there is a fine opening from which a 

 canal leads to the kidney. This renopericardial 

 opening and tlie canal are difficult to see witii the 

 unaided eye. 



THE VISCERAL MASS 



The \ery short esophagus enters tiie large and 

 somewliat twisted stomach, into which a wide sac 

 containing tlie crystalline style also opens. The 

 visceral nuiss (figs. 72, 7.'3) occupies the dorsal 

 half of tlie body, above the adductor nmscle. It 

 consists of esophagus, stomach, crystalline style sac 

 inside the pyloric process, and intestine embedded 

 in connective tissue. The stomach directly com- 

 municates with the digestive diverticula, a green- 

 ish mass of glandular tissue (fig. 73, St., di. d.), 

 which completely surrounds both stomach and 

 intestines. The intestine, after leaving the stom- 

 ach, makes a loop (fig. 73, in., and fig. 197, ch. X) 

 which ends in the rectum (fig. 71, r.) at the dorsal 

 edge of the adductor muscle. A snuiU rosette at 

 the tip of the slightly protruding rectum surrounds 

 the anus (fig. 72, an.), located in the area con- 

 tinuall}' swept by the current of water from the 

 cloaca (fig. 72, cl.). 



Between the digestive diverticula and the 

 surface epithelium lie the gonads (fig. 73, gn.). 

 After spawning these layers disappear almost 

 completely, being represented only by a thin 

 germinal lining. The gonad is not visible to the 

 unaided eye at this stage. At the time of full 

 sexual development the layer of gonadal tissue in 

 large specimens may reach several millimeters in 

 thickness. Manj^ branching channels, the gono- 

 ducts (fig. 72, gd.), through which sex cells are dis- 

 charged, are clearlj' visible on the surface of a 

 sexually mature specimen. They all empty into 

 a common gonoduct leading into the urine >- 

 genital groove (fig. 72, ur. \-.) from whicli eggs or 

 sperm ai-e discharged into the epibranchial cham- 

 ber (ep. br. ch.). Secondary sex cliaracters are 

 absent. 



The sex of the oyster can be recognized by 

 niicrosocopic examination of thegonad. Hermaph- 

 rodites among adult C. virginica are rare. Out of 

 many thousands of oysters examined in the course 

 of my studies I have found only one oyster with 

 tlie gonads containing both eggs and sperm. The 

 European 03'ster, (>. edulis, and tiie Olympia 

 oyster, (>. Iiirida, are hermaphroditic (A full 



discussion of sex in the ovster is found in chapter 

 XIV.) 



The position of the fully developed gonad in 

 relation to other organs of the visceral mass can 

 best be studied in a series of trans\'erse sections of 

 the dorsal half of the oyster. Figure 73 shows the 

 relative position of the organs as seen on the sec- 

 tion made just below the labial palps. The 

 gonad (gn.) is irregular in shape and located close 

 to the surface of the body. The digestive divertic- 

 ula (di. d.) occupy the larger part of the visceral 

 mass between the gonad and the digestive tract 

 itself, which at this level is represented by the 

 stomach (st.) and two cross sections of the in- 

 testines (in.). The rest of the visceral mass 

 consists of connective tissue (c.t.) containing 

 irregular blood sinuses (bl. s.), which may be full 

 of blood. The series of twisted tubules compris- 

 ing the kidney (k.) is located near the surface on 

 both sides of the body above the gills. 



The large empty chambers between the gills 

 and the visceral mass directly communicate with 

 the water tubes (w.t.) of the gills (g.). The 

 epibranchial chamber (ep. br. ch.) on the left side 

 is much smaller than the corresponding chamber 

 on the right side. The latter, called by Nelson 

 (1938) promyal chamber (pr. ch.), extends to the 

 dorsal end of the oyster and opens to the outside 

 in the posterodorsal part of the body, independ- 

 ently of the cloaca. Water tubes (w.t.) inside the 

 gill plates open into these chambers. Soft and 

 delicate tissue of the gills is supported by the 

 framework of chitinous rods, the largest being 

 located at the base of the gills (g.). Two sets of 

 muscles (g.m.), one below and one above the 

 largest gill rods, control the mo\ements of the 

 plates. The five principal blood vessels of the 

 gills are located above the skeletal rods: the com- 

 mon afferent vein (c. af. v.) ; two branchial efferent 

 veins (br. ef. v.), one on each side; and two lateral 

 afferent viens (1. af. v.), also one on each side 

 of the oyster. 



NERVOUS SYSTEM 



The nervous system can be studied best by 

 reconstructions made from sectioned material 

 since only the principal ner\'es and ganglia can 

 be revealed by dissection. The \isceral ganglion 

 (fig. 72, v.g.) is located in a slight depression on 

 tlic anterior side of the adductor muscle, partially 

 hidden Ijy the tip of the pyloric process. It can 

 be observed by cutting the wall of the cloaca and 



GENERAL ANATOMY 



71 



