80 GENERAL PRINCIPLES OF ZOOLOGY 



oxygen is taken up by the bluish hcemocyanin, which contains a trace of copper: 

 in the Sipunculids by hcemoerythrin, etc. The blood-plasma, as a rule, is the 

 seat of the color (Chironomtts, Hirudinea, earthworms, and most other annelids); 

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

 Solcn and some other molluscs, and also mPhoroiiis. Colored elements contain- 

 ing haemoglobin, identical with blood-corpuscles, are found besides in the 

 ccelomic fluid of many annelids, and in the ambulacral vessels of some echino- 

 derms. Most widely distributed in the invertebrates are the leucocytes, dis- 

 tinguished 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. 



Muscular Tissue 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 between the smallest 

 particles. Muscles and hence their separate elements, the 

 muscle-fibres and muscle-fibrils, on the contrary, can shorten 

 ^Jj only by correspondingly increasing in diameter (fig. 48); 

 they can therefore accomplish motion only in a definite 



^^J direction, that of the axis of the muscle. The muscle- 



" . SSMHEHB 



, H| substance consequently is more limited in its movement 



than is protoplasm, but on the other hand it has the 

 uu* 

 FIG g _ advantages of greater energy and greater rapidity. One 



Four striped is able to decide with considerable accuracy, from the 



muscle fibres intensity and rapidity, whether in a given case a movement 

 in resting and " 



contracted has been brought about by the agency of protoplasm or by 



condition muscle-substance. 

 (after Rollet). 



Structure of Muscle Substance. These physiological 



considerations show that protoplasm and the contractile substance are 

 morphologically different, and that therefore one must distinguish sharply 

 between formative cells, or muscle-corpuscles, and the product of these 

 cells, the contractile substance, just as in the case of connective tissue, 

 between the corpuscles and fibrils. There are recognized two kinds of 

 muscle-substance, the homogeneous, or smooth, and the cross-striated. 

 Since the former looks very similar to non-granular protoplasm, the 

 boundary-line between it and the muscle-corpuscle is more difficult to 

 recognize than in the case of the latter, which in its minute structure is 

 quite different in appearance from protoplasm. In cross-striated muscles 

 the contractile portion consists of two substances regularly alternating 

 with one another in the direction of the contraction of the muscle, of which 

 the one is doubly, the other singly, refractive (figs. 25, 48, 51). 



