152 BULLETIN OF THE BUREAU OF FISHERIES. 



p. 153). It receives blood from the area of the mantle lying close to the border through 

 a network of fine vessels that empty into it throughout its course. 



The ascending pallial veins are a series of vessels covering the internal face of the 

 mantle extending upward, more or less parallel to each other, from the margin of the 

 mantle to the horizontal vein that runs parallel and just below the line of attachment 

 of the gills (fig. 134, APV). They result from the union of numerous fine capillary 

 vessels that form a network all through the mantle and represent the principal channels 

 by which the blood leaves the mantle. 



The Iwrizontal veins are paired vessels which follow a sinucfus course the length of 

 the mantle, parallel to and just below the roots of the gills (fig. 134, HV). Beginning 

 anteriorly as a vessel of small diameter, each horizontal vein gradually increases in 

 diameter as it runs backward. In the region of the posterior adductor muscle it reaches 

 its maximum size and is quite conspicuous. Throughout its entire course it is connected 

 with the ascending pallial veins which discharge their blood into it. Near the posterior 

 end it receives the anastomosing vein and finally connects with the marginal sinus 

 behind the posterior adductor muscle. 



The intermuscular sinus is also a paired vessel. It arises between the palps at their 

 point of attachment and extends back over the muscles of the foot and byssus in a 

 series of cavities. One branch of the sinus lies between the anterior retractor muscles; 

 a pair of vessels runs laterally to these muscles, extending back as far as the posterior 

 adductor muscle; and still another pair goes between the posterior retractor muscles. In 

 their course they receive veins from the liver, foot, kidney, and the mesosoma (fig. 135, 

 /A/5). 



The mesosomal veins arise as three main trunks on each side of the mesosoma. A 

 median vessel runs posteriorly close to the free border. Above it, two lateral vessels run 

 backward and unite with it. The common trunk, thus formed, empties into a transverse 

 sinus on the anterior ventral side of the posterior adductor muscle, which connects with 

 the longitudinal vein and vessels of the kidney (fig. 135, MV). 



The visceral veins include numerous small vessels which convey blood from the 

 liver, stomach, intestines, etc., chiefly to the network of vessels within the kidney. The 

 blood supply of the liver is particularly rich and involves a complicated mass of vessels 

 which envelop the lobes. The blood from the dorsal and deeper parts of the liver is 

 carried directly into the kidney, while that from the ventral portion and superficial area 

 is conveyed to the kidney by way of the internal plicate canals (fig. 135, 1 PIC). A small 

 amount of blood from the surface of the liver is carried off by small vessels that empty 

 into the afferent vein of the gills. 



The branchial veins on each side of the body consist of a single afferent branchial vein 

 at the roots of the gills and a pair of efferent branchial vessels which border the free ends 

 of the reflected filaments (fig. 134, EBV, and fig. 135, ABV and EBV). The afferent 

 branchial vein is connected with vessels of the kidney from which blood is received and 

 the efferent branchial veins open anteriorly into the anterior longitudinal vein near the 

 base of the palps. 



The longitudinal veins are paired vessels more or less enveloped by the kidney tissue. 

 They extend from the base of the posterior adductor muscle to the anterior extremity of 

 the gills. The position of the vessel on the right side of the body is indicated by a 

 dotted outline in figure 133, A LV and PLV, page 149. Sabatier (1877), who named the 



