VASCULAR SYSTEM OF THE LOBSTER 175 



tips (Fig. 70). Each gill, in turn, is drawn out into innumerable 

 flaps or lamellae, closely packed together like leaves of a book. 

 In each gill there are two blood vessels, one ventral, one dorsal. 

 The blood from the body cavity enters the ventral vessel, pass- 

 ing into the gills; here capillary vessels branch into the lamellae, 

 and are continuous with similar capillaries emptying into the 

 dorsal vessel. In the gills, therefore, there is a small closed 

 circulation from one venous ventral tube into another dorsal 

 tube, in which it passes toward the heart. The thin-walled 

 lamellae of the gills are in contact with water, which passes 

 through the branchial chamber by activity of the scoop or 

 scaphognathite, which consists of the fused bract and exopodite 

 of the second maxilla (Fig. 68). The blood is thus brought in 

 contact with fresh water and is aerated, giving off C02, and 

 taking oxygen before passing to the dorsal branchial tube. 



Various parts of the body wall are drawn out to form these 

 triangular gill pockets. Some are on the appendages and are 

 termed podobranchs ; others are on the basal joint of the append- 

 age and do not come out when the appendages are removed. 

 These are termed arthrobranchs, from their position on the 

 joints; still others originate on the body wall itself, and are 

 termed pleurobranchs or side-wall gills. The number of each 

 kind gives the basis for a gill formula, which differs with each 

 species of Crustacea; the formulae for the lobster and the cray- 

 fish are given below: 



Podobranchiae Arthrobranchiae Pleurobranchiae 



Crayfish (Astacus) . . . .6 n 3 (2 rudimentary) 



Lobster (Homarus) . . .6 10 4 



After aeration in the gills, the blood is slowly passed on into 

 large branchial sinuses (branchio-cardiac sinuses), which lead 

 into the pericardial chamber containing the heart. The latter 

 has the form of a pentagonal shield with six openings or ostia, 

 of which one pair is dorsal, one lateral, and one ventral. Blood 

 from the pericardium enters the heart through these ostia and 

 is prevented from going back again by valves on the inside, 

 which are closed upon pressure due to contraction of the heart. 

 This pressure forces the blood out into arteries as follows: 



