IO8 GENERAL PRINCIPLES OF ZOOLOGY. 



Solen Icguincn, Tellina planata, Pcctnnculus glycimeris, and 

 others; among the annelids in the Capitcllidce, Glycera, 

 Polycirrus, Lcprea, leeches, and cartJiivorms ; among insects 

 in Chironomus). Often other coloring matter occurs instead 

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

 lobster, the oxygen is taken up by the bluish haemocyanin, 

 which contains a trace of copper; in the Sipiincnlids by 

 haemoerythrin, etc. The blood-plasma as a rule is the seat 

 of the color (Chironomus, Hirudinca, earthworms, and most 

 of the 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 

 PJioronis. Colored elements containing haemoglobin, 

 identical with blood-corpuscles, are found besides in the 

 coelomic fluid of many annelids (Capitellidcz, Glycera, 

 Leprca, Polycirrus], and in the ambulacral vessels of echino- 

 derms (Ophiactis vircns, certain holothurians). Most widely 

 distributed in the invertebrated animals are the leucocytes, 

 which are distinguished by their active amoeboid move- 

 ments; still even these may be absent, and then the blood 

 is a fluid without any organized corpuscles. 



Relations of Plasma to Corpuscles. The blood has 

 often been called a connective tissue having a fluid inter- 

 cellular matrix. This conception is neither physiologically 

 nor morphologically justifiable. For, if we leave out of 

 consideration the utterly different function of the blood, 

 it still cannot be shown that the blood-plasma is a product 

 of the blood-cells, as the intercellular substance is a prod- 

 uct of the cells of the connective tissue. The occurrence 

 of blood without cells is rather a proof that the formation 

 of blood-plasma takes place independently of the blood- 

 corpuscles. 



3. Muscular Tissue. 



Marked Characteristics of Muscular Tissue. Most 

 sharply characterized functionally is the muscle-tissue, 

 inasmuch as it is the agent of active movements in the 

 animal body. Since active mobility occurs in protoplasm, 



