so 



GENERAL PRINCIPLES OF ZOOLOGY 



oxygen is taken up bv the bluish Innimcyaiiiii, which contains a trace of copper; 

 in die SipuncLilids bv luciiiocrvlhriii, etc. The blood-plasma, as a rule, is the 

 seat of the color [Cliinyiionnis, Hirudinea, earthworms, and most other annelids); 

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

 Solen and some other molluscs, and also mPlioroiiis. Colored elements contain- 

 ing ha?moglobin, identical with blood-corpuscles, are found besides in the 

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

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

 tinguished bv their active amceboid movements; still even these may be absent, 

 sal then the blood is a tluid without any organized corpuscles. 



3. Muscular Tissue. 

 Muscular Tissue is the agent of active mo\ements in tlie animal body- 

 Since active mobility occurs in protoplasm, it is important to notice the 

 differences between the two kinds of moxement. 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 



miisdc-fibrcs and musdc-fibrih, on the contrary, can shorten 

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

 they can therefore accomplish motion only in a definite 

 tlirection, that of the axis of the muscle. The muscle- 

 substance consequently is more limited in its movement 

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

 advantages of greater energy and greater rapidit\'. One 

 is able to decide with considerable accuracy, from the 

 intensity and rapidity, whether in a given case a moyement 

 :ontra"ctc(l has been Itrought about by the agency of protoplasm or by 



condition muscle-substance. 



(after RolK't). 



Structure of Muscle Substance. — These physiological 

 considerations show- that protoplasm and the contractile sulistance are 

 morphologically different, and that therefore one must tlistinguish sharply 

 between formative cells, or miisclc-corpMSi-Ics, and the product of these 

 cells, the contracti'c substance, just as in the case of connective tissue, 

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

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

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

 boundary-line between it and the muscle-corpuscle is more dilficult 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 anolher in (he direction of the contraction of the nuiscle, of which 

 tlie one is doubly, the other singly, refracli\-c (figs. 2^,, 48, si). 



Fig. 48.— 

 Four striped 

 muscle fibres 

 in resting; and 



