90 GENERAL PRINCIPLES OF ZOOLOGY. 



Lymph is distinguished from blood by the entire lack of red 

 blood-corpuscles and the slight coagulability of its plasma. 

 Lymph is accordingly a proteid-containing fluid with leucocytes, 

 which are here called lymph-corpuscles. 



In the majority of invertebrated animals there is present only 

 one kind of nutritive fluid, and not even this in every class; the 

 fluid is called blood, although it is usually colorless. Where 

 color is present, it is generally, if not always, a yellowish red or 

 an intense red; this may, even as in the vertebrates, be caused by 

 haemoglobin (among the molluscs in Planorbis, Area tetragona, 

 A. now, Solen legumen, Tellina planata, Pectunculus glycimeris, 

 and others; among the annelids in the Capitellidae, Glycera, 

 Poly cirrus, Leprcea, leeches, and earthworms; among insects in 

 Chironomus). Often other coloring matter occurs instead of 

 haemoglobin : in the cuttlefish, many snails, and in the lobster and 

 Limulus, the oxygen is taken up by the bluish haemocyanin, which 

 contains a trace of copper; in the Sipunculids by haemoerythrin, 

 etc. The blood-plasma, as a rule, is the seat of the color (Chiro- 

 nomus, Hirudinea, earthworms, and most other annelids); only 

 exceptionally do colored blood-corpuscles occur, as in the case of 

 Area, Solen, and the other mussels mentioned above, and also in 

 the genus Phoronis. Colored elements containing haemoglobin, 

 identical with blood-corpuscles, are found besides in the ccelomic 

 fluid of many annelids (Capitellidae, Glycera, Leprea, Polycirrus), 

 and in the ambulacral vessels of echinoderms (Ophiactis virens, 

 some Holothurians). Most widely distributed in the invertebrate 

 animals are the leucocytes, which are distinguished by their active 

 amoeboid movements; still even these may be absent, and then the 

 blood is a fluid without any organized corpuscles. 



3. Muscular Tissue. 



Characteristics of Muscular Tissue. Most sharply character- 

 ized functionally is the muscle- tissue, inasmuch as it is the agent 

 of active movements in the animal body. Since active mobility 

 occurs in protoplasm, it is important to notice the differences 

 between the two kinds of movement. The distinctions lie in 

 the direction and in the intensity of the movement. A mass of 

 protoplasm has the capacity to move hither and thither in all 

 directions, because in it there is a high degree of mobility be- 

 tween the smallest particles. Muscles and hence their separate 



