BLOOD SYSTEM. 



403 



course, quite different. Like the crustacean, also, the 

 heart is a systemic heart — i. e., aerated blood fills the 

 heart and is distributed to the tissues. 



Gastropods, Univalves. — Take, for example, a 

 snail. These are air breathers. As already said (page 

 340), there are four openings of the body in front. 

 These are (1) the mouth, (2) the genital opening in the 

 immediate vicinity, (3) the vent, and (4) the pulmonic 

 opening beneath the shell on the right side. We are 

 concerned now only with this last. It opens into a sac 

 immediately beneath the anterior upper portion of the 

 shell (pulmonic sac), on the interior of which are pro- 

 fusely distributed capillary blood vessels. 



Breathing. — The change of air seems to be effected 

 by a muscular arched membrane just beneath the lung 

 sac, which may be compared- to a diaphragm. The con- 

 traction of this membrane lowers the arch, expands the 

 lung sac, and draws in the air. This contraction, of 

 course, compresses the viscera, 

 on which it rests. On the relaxa- 

 tion of the membrane the natural 

 elasticity of the compressed vis- 

 cera lifts the arch and expels 

 the air. 



' Circulation. — The heart has 

 three chambers, two auricles and 

 a ventricle. Contraction of the 

 ventricle throws the blood to the 

 tissues, whence it is gathered into 



the sinuses, and thence it is again taken up and dis- 

 tributed over the inner surface of the lung sac, where 

 it is aerated and then returned to the heart, to be dis- 

 tributed again to the tissues as aerated blood. 



We have taken the higher air-breathing forms of 

 snails and slugs, but in water breathers, of course, we 



Fig. 283. — Diagram of heart 

 and branchiae of a Gastro- 

 pod : v, ventricle ; au, au- 

 ricle ; br y branchiae. 



