Chap, v.] BLOOD CORPUSCLES. 183 



and forty thousand to two hundred and thirty thou- 

 sand (Malassez). 



In Amphioxus and the Urochordata, there are 

 no red blood corpuscles, and, as a rule, these are not 

 found in what have been called invertebrates ; they 

 have, however, been observed in Solen (a Mollusc) ; 

 Glycera (a Chsetopod) ; Amphiporus (a Nemertean) : 

 and Phoronis (a Gephyrean). 



In most Invertebrates the blood corpuscles 

 are either limited to the fluid in the body cavity, or 

 are found more or less numerously represented in 

 the fluid contained in the vessels. In all these cases 

 they are single cells, generally amoeboid in character, 

 and they vary considerably in size and number. 



In Echiiioderms, Arthropods, and Molluscs, 

 the corpusculated fluid is contained in a system of 

 more or less completely closed walls (vide infra) ; in a 

 large number of worms the fluid in the vessels is, on 

 the other hand, said to be non- corpusculated, and, at any 

 rate where corpuscles are found, they are often, as in 

 the earthworm, rare, and of small size (-g-oVo i jicn j 

 Lankester). It is, however, only quite recently that 

 observations have been directed to the presence of 

 corpuscles in the blood of Annulata, and since then 

 they have been observed in several members of the 

 group (Eunice, Nereis). 



BLOOD-VESSELS AND HEARTS. 



In all the forms already mentioned, part of the 

 blood at least is contained in a system of more or less 

 completely closed vessels, by means of which it is 

 conveyed from part to part of the body ; these vessels 

 make up the blood- vascular system. When best 

 developed this system has, on some parts of its course, 

 a contractile organ, by means of which the fluid is 

 pumped along ; this is the heart. In the Vertebrata, 

 the vessels given off from the heart (arteries) do not, 



