IN RELATION TO ZOOLOGICAL DISTINCTION. 63 



These zoological differences are paralleled in the invertebrates, as will 

 be manifest even after a most superficial inquiry, as, for instance: In the 

 Protozoa and Porifera there is an entire absence of any fluid which we are 

 justified in regarding as being even an analogue of the blood. In the Hy- 

 drozoa, Actinozoa, and Echinodermata the perivisceral or chyliferous fluid 

 is the simplest expression of a rudimentary blood. In the former this fluid 

 scarcely differs from that in which the organism lives, and it contains but 

 few if any corpuscles, which are of a rudimentary character. In the Actin- 

 ozoa and Echinodermata there is a distinct approach to a typical blood, 

 there being both rudimentary and typical cells. The chylaqueous fluid 

 of the Annelida contains typical corpuscles, and it is among the animals 

 of the annuloid series, in Trichoscolices and Annelida, that we find the first 

 appearance of hemoglobin, of colored corpuscles, and of a true blood i.e., 

 a circulatory fluid which combines the functions of both circulation and res- 

 piration, and therefore that is comparable with the blood of the vertebrate. 

 The first appearance of hemoglobin is, as far as known, in holothuridean 

 (Thyonella gcmmata}, ophiuridean (Ophiactis virens), and turbellarian 

 (Polia sanguimbra) echinoderms, although histohematins have been found 

 in certain of the Porifera and Actiniidce. The first appearance of an ana- 

 logue of the erythrocy te has been noted in the Gephyrea (Sipunculus nudus, 

 S. balanorophus, S. echinorhynchus, Phascolosoma elongatum), in which are 

 corpuscles having a distinct cell wall which incloses a colored fluid in which 

 a nucleus is suspended. The coloring matter of the corpuscles is allied to 

 hemoglobin, but the corpuscles have no histological relationship to the 

 vertebrate erythrocyte. The fluid of the pseudo-hemal system of certain 

 Annelida contains a red coloring matter in the form of a histohematin which 

 is closely allied to hemoglobin chemically and physiologically; that of 

 others is green and colored by chlorocruorin, which also is closely allied to 

 hemoglobin; and that of others is colored by hemoglobin, etc. 



In invertebrata the blood or pseudo-blood may be colored or color- 

 less, which differentiation is not related to the position of the organism 

 in the scale of life; but in all of the non-generate vertebrates the blood is 

 colored. In the invertebrates the coloring matter may be in solution in the 

 circulatory fluid or in the corpuscles, but, as a rule, it is in the fluid, whereas 

 in the vertebrates it is without exception in the erythrocytes. While the 

 corpuscles of invertebrate blood are allied histologically to the leucocytes 

 of vertebrate blood and not to the erythrocytes, when colored they may be 

 (but usually are not) respiratory like the erythrocytes. 



In the vast majority of invertebrates the coloring matter of the blood 

 is hemocyanin, which is, as far as known, invariably in solution in the 

 plasma, the venous blood being colorless, or nearly so, and the arterial blood 

 of various tints of blue or violet. Various other pigments have been found, 

 as shown in the preceding chapter. In no instance has it been recorded, 

 as far as we are aware, that both hemocyanin and hemoglobin coexist in 

 the same blood or even in the same individual, although in certain organisms 

 with a hemocyanin blood histohematins and myohematins have been found 

 in certain of the body structures. Bloods that owe their coloration to other 



