CIRCULATION OF THE BLOOD. 



493 



differentiation of the blood-vascular system is observable. In some of 

 the lowest forms, as in polyzoa, neither a contractile heart nor even 

 vessels can be detected; circulation in them, as in lower forms, being 

 carried on by mere imbibition. In the tunicata the heart, whose 

 position closely resembles its ventral situation in the vertebrata, has 

 no valves between its dilated chambers, and the blood is propelled by 

 opposite peristaltic movements, first in one direction and then in 

 another ; hence, here the heart is sometimes systemic and sometimes 

 respiratory. The most perfect form of circulation found in thd 

 mollusca exists in the cephalopoda. In them there is a systemic 

 ventricle provided with valves at its orifice, with systemic arteries, the 

 blood being returned into a large venous sinus, from which it passes to 

 the gills through contractile vesicles, the branchial hearts, which serve 

 to propel the blood through the gills ; from there it passes again into 



H 



Fig. 181.— Diagrams of the Great Blood-Vessels in the Fresh-Water 



Mussel and the Fish. (Jeffrey Bell.) 



C, fresh-water mussel : H, heart : AA, anterior, and PA. posterior aortse ; A, auricle. D, fish : H, 



heart; BR, -branchial vessels ; DA, dorsal aorta. 



contractile venous sinuses, which, therefore, act as auricles, and is then 

 driven to the heart. 



Thus we find that in the invertebrata the circulatory apparatus, even 

 in the highest forms, contrasted with what we shall find in the vertebrata, 

 does not consist of a continuous series of tubes, but that the blood 

 passes from such vessels into spaces (lacunas or perivisceral spaces) 

 without distinct walls. Connected with the vessels we often find several 

 pulsating cavities more analogous to the lymphatic or venous hearts 

 found in the vertebrata than to a true respiratory or systemic heart. 

 When a heart is present in the invertebrates, it is single, is, as a rule, 

 placed on the dorsal aspect of the body, contrasted with its ventral 

 position in vertebrates, and is of a systemic and not respiratory function. 

 In the invertebrata there is no trace of a portal system, the liver being 

 supplied by the systemic arteries. 



