356 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



Fig. 238. — Blood corpuscles 

 of a crustacean. (After 

 Gegenbaur. ) 



amphioxus, which is very doubtfully a vertebrate) is red. 

 Below this — i. e., in invertebrates — the blood is not red.* 



3. Higher Invertebrates. — Among these we include 

 arthropods, worms, mollusca, and echinoderms — i. e., 



all above ccelenterates ; in other 

 words, all that have a blood sys- 

 tem and a true blood at all. In 

 these the red corpuscles are 

 wanting; only white corpuscles 

 are found (Fig. 238). In these, 

 therefore, the blood is colorless 

 or nearly so. This kind we shall 

 call invertebrate blood. 



4. Ccelenterates. — These include medusae and polyps. 

 In these, as already said (page 342), we have no true 

 blood differentiated from digested food, nor blood sys- 

 tem differentiated from the digestive system. The di- 

 gested food carried by ciliary circulation is the nutrient 

 fluid. This, therefore, may be called food blood, or ccelen- 

 terate blood. 



5. Protozoa. — In these there is no circulating fluid of 

 any kind. 



The following schedule briefly expresses these facts : 



Mammalian ) 



. . \ Blood — red, non-nucleated, 



viviparous. ) 



2. Oviparous vertebrate blood — red, nucleated — birds, rep- 

 tiles, amphibians, and fishes. 



3. Invertebrate blood — white, nucleated — arthropods, 

 worms, mollusca, echinoderms. 



4. Ccelenterate blood — white, no globules, food blood— 



medusae, and polyps. 



5. Protozoa — no circulating fluid. 



Embryonic Development of Blood. — Some of the 

 above phases are found in the embryonic development 



Blood 

 red. 



Blood 

 color- 

 less. 



J 



* Some worms have a kind of red blood. 



