94 



FIRST LESSONS IN ZOOLOGY 



circuhitai-)' s)'stcm consists of definite tubes ; these open 

 into loose ill-defined spaces in the body-cavity. In such 

 the blood is moved gradually throughout the animal, but 

 not so definitely and quickly as in others 

 where the blood runs in definite vessels. 

 In the earthworm there is no such heart 

 as in higher animals, but the blood- 

 vessel along the dorsal line and some 

 of its branches around the sides have 

 muscular walls and " beat " by a wave of 

 contraction running toward the head. In 

 insects the dorsal blood-vessel beats in 

 the same way, but generally more vigor- 

 ously. In the young larva of a mosquito 

 Fic, c8 — Dii^rim °'' "y^ph of a May-fly with transparent 

 <if circul:itoi-y skin the beating can be easily seen under 



system of VDUiii,' ... t n ii • 



dragon-fly- i ii ^'ic microscope. In molluscs there is a 

 middle is tlie well-developed heart; it can be well seen 



chambered dor- . . ^ . , ^_,, 



sal vessel or lieart m the iresh-watcr mussel. I he Crustacea 

 with single ar- gjgQ ]-,ave a heart. This can be seen at 



tery ; arrows in- 

 dicate direction work in a water-flea under the microscope, 



V(il'""'l-'^n'^'^^"'^' or can be readily demonstrated in a crab or 



(After Kolbe. ) ■' 



crayfish killed with chloroform or ether. 

 In vertebrates the blood circulates in a definite system 

 of tubes through which it is pumped by a heart. The 

 fishes (fig. 59) have the heart consisting of two parts, with 

 muscular •\\'alls, a single auricle and a single ventricle. 

 Tlie auricle receives the blood pouring from all the tissues 

 of the body through the veins. It contracts and forces 

 the blood into the ventricle. This then contracts and 

 drives it into a short vessel called the ventral aorta, 

 which gives off a branch artery for each gill-arch. The 

 gill-arteries divide up into ca])illaries in the gills, whence, 

 after aeration, the blood is gathered by another arter}' and 

 carried to the dorsal aorta, from which branch arteries 



