154 BULLETIN OF THE BUREAU OF FISHERIES. 



vessel, divided it into two parts. The portion lying in front of the oblique vein was 

 termed the anterior longitudinal vein, while that part which is behind and really con- 

 tinuous with the oblique vein was called the posterior longitudinal vein. The longitudinal 

 vein is smallest at the anterior extremity and increases in diameter in its backward 

 course. It is the central sinus that receives blood from all parts of the body and from 

 which it is transferred directly to the heart. 



The anastomosing veins are a pair of short trunks that establish connections between 

 the horizontal vein, transverse sinus of the posterior adductor muscle, and the posterior 

 longitudinal vein (fig. 134, AnV, p. 153). The vessel on each side arises from the hori- 

 zontal vein near its junction with the marginal sinus, runs obliquely forward and upward 

 to the ventral side of the posterior adductor muscle, where it connects with the transverse 

 venous sinus and also with veins from the anal membrane, and from there continues 

 forward to the kidney, where it terminates in the posterior longitudinal vein. 



The afferent oblique veins of the heart are prominent oblique vessels on each side of 

 the body which extend downward and backward from the pericardium to connect 

 with the longitudinal vein (fig. 133, AOV, p. 149). They represent the final channel by 

 which the blood reaches the heart. The oblique vein on each side is inclosed in the 

 renipericardium canal, to which it is attached on the dorsal wall. The anterior and 

 ventral surfaces are more or less covered with folds of kidney tissue similar to that of 

 the pericardial glands which envelop the auricles and with which it is continuous. 

 This portion of the oblique vein is free from any attachment and lies submerged in the 

 fluid of the renipericardium canal. 



blood. 



The blood, or haemolymph, is a clear, transparent fluid except for a slight opalescent 

 tinge. The corpuscles which are suspended in it are colorless, amoeboid cells or lympho- 

 cytes. If a drop of the haemolymph is placed under a cover glass on a microscopic slide 

 and examined under a microscope, the lymphocytes exhibit considerable activity. 

 The simplest form they assume is a sphere, but, being capable of pronounced and con- 

 tinuous amoeboid movement, they take on all sorts of shapes from the sphere through 

 the types with few pseudopodia to stellate forms with many slender, conical-shaped 

 pseudopodia as shown in figure 136, a, b, c. During these movements the nuclei remain 

 unchanged in form. When exposed to the air the amoebocytes collect together in 

 tangled groups (fig. 136, c), after which the central mass runs together as a Plasmodium. 

 This phenomenon seems to be of great significance, for since the blood of lamelli- 

 branchs contains no fibrin and therefore is incapable of clotting, it is probable that 

 this property of the corpuscles to form a plasmodium takes the place of the fibrin clot. 



In size the corpuscles vary from 8 to 12 microns in diameter, with an average of 

 about 10 microns. In histological preparations the nuclei are quite large and promi- 

 nent; they contain one or two nucleoli and many chromatin granules (fig. 137). The 

 cytoplasm appears as a fine reticulum or presents a uniform granular appearance. The 

 number of corpuscles present in a given volume of haemolymph is small when compared 

 to the number of corpuscles present in the blood of vertebrates. 



The fluid portion of the haemolymph is an albuminous, salty liquid with an osmotic 

 concentration equal to that of the sea water. When heated, a slight coagulation occurs. 

 It is precipitated by picric acid, nitric acid, and mercuric chloride. It gives a decided 



