132 MORPHOLOGY OF INVERTEBRATE TYI'KS 



sinuses. The heart is inclosed in a pericardium. It is situated 

 immediately under the carapace in the posterior region of the 

 latter. The heart has a peculiar shape, being flattened above, 

 rounded below and somewhat wider in front than behind. It is 

 perforated by three pairs of openings called ostia, through which 

 the blood from the pericard is admitted to the heart and which 

 are provided with valves to prevent the return of the blood to 

 the pericardial sinus. One pair of the ostia is dorsal, one lateral 

 and one ventral. The heart gives off five arteries in front and 

 two behind. The anterior arteries are a single anterior aorta or 

 median cephalic artery which supplies the brain and eyes, two 

 antennal arteries which supply a number of organs, and two 

 hepatic arteries. Posteriorly the heart gives off the posterior 

 aorta or median dorsal abdominal artery which runs backward 

 above the intestine, giving off six pairs of lateral arteries to the 

 pleopods, and the sternal artery which runs down, passes through 

 the nerve cord and divides into two branches. The anterior 

 branch is called the subneural thoracic artery and runs straight 

 forward under the nerve cord. The posterior branch is called the 

 subneural abdominal artery and runs backward under the nerv- 

 ous cord. The arteries split into smaller branches, become finally 

 capillaries and open into lacunae. From here the blood, which is 

 colorless and contains amcebocytes, passes to the ventral sinus 

 which surrounds the nervous cord, and is driven to the gills by 

 the afferent branchial vessels. Having been oxidized in the capil- 

 laries of the gills the blood passes through the efferent branchial 

 vessels into the branchio-cardiac veins. There are altogether 

 five pairs of these veins opening into the pericardial sinus. 



Respiratory system. The respiratory system of the lob- 

 ster consists of twenty pairs of gills. Of these six pairs are podo- 

 branchicB attached to the appendages of the eighth to thirteenth 

 somites (second maxilliped to fourth pereiopod), ten pairs are 

 arthrobranchi(E attached to the articulating membrane of the 

 ninth to thirteenth somites, and four pairs pleurobranchice at- 

 tached to the endopleurites of the eleventh to fourteenth somites. 



