VENUS MERCEXARIA LINNAEUS 



213 



and lacunae between the various organs, i. e., by means of spaces 

 devoid of a lining of their own. The heart contains arterial blood 

 and is composed of a single ventricle and two auricles. It is 

 inclosed in the pericard on the dorsal surface of the animal. 

 The ventricle has the shape of a pyramid with the apex pointing 

 anteriorly, and is perforated by the rectum. The auricles are 

 quite thin-walled and capable of considerable distension. The 

 ventricle gives rise to two aortas. The anterior aorta runs forward 

 above the alimen- 

 tary canal and 

 soon splits up into 

 several arteries; 

 the posterior aorta 

 runs backward 

 below the rectum 

 and soon divides 

 into two posterior 

 m a n tie arteries. 

 The posterior 

 aorta forms a pul- 

 sating bulbus ar- 

 teriosus not far FIG. 42. Diagram of the structure of a gill of 

 from the ventricle A"d nta c yS> iea after Peck, from Parker and Has- 

 well's Textbook of Zoology, b. v, blood vessels; /, 



OI the heart and branchial filaments; /. /. j, interlamellar junction; i. 

 still in the pericar- / j, interfilamentar junction; os, external ostium; os', 

 dial cavity The mternal st i um ; '^ water tubes. 



blood passes from the lacunas between the various organs into a 

 venous longitudinal sinus situated under the pericard, thence 

 through a network of veins in the nephridia into the branchial 

 arteries. After oxidation in the gills the blood enters the bran- 

 chial veins and returns to the heart through the auricles. A 

 very small quantity of the blood runs in other channels than 

 those referred to above and returns to the heart either venous 

 or partly oxidized in the mantle. The blood itself contains 

 amcebocytes and is rich in albuminous substances. 



