9 8 



HISTOLOGY 



This structure has but weak powers of contraction and might be con- 

 sidered as half connective tissue rather than muscle (Fig. 94). 



Cerebratulus shows an interesting form of fiber in which the fibrils 



' "~~-^=^=^^ 



FIG. 94. Two contractile fibers from Euspongia. (From SCHNEIDER after F. E. SCHULTZE.) 



are few and in another part of the cell from the nucleus. See the descrip- 

 tion of the blood vessels in this form for descriptions and illustration 

 (Chapter XII). 



The longitudinal muscle fibers of A scar is megalocephala show a very 

 highly specialized cell in which the nucleated cell body is situated on 



the inner side of 

 its plate-shaped 

 muscle bundles. 

 The bundles con- 

 sist of a single 

 row of fibers em- 

 bedded in a 

 cement substance 

 of considerable 

 thickness. The 

 cell body sends 

 off some peculiar 

 processes, one of 

 which is said to 

 from the common 



mus.fi. 6. 

 /\ 



FIG. 95. Transection of body-wall muscle cell of an Ascaris from 

 the cat's pylor/s ; nu., nucleus ; mus.fi.b., muscle fiber-bundles in 

 transverse section. X 1000. 



have a nervous function. Figure 95 was drawn 

 Ascaris in the cat. 



The gizzard of the earthworm is provided with a covering of very well- 

 developed and closely packed fibers of rather short length. Their sides 

 are flattened from close contact with their fellows and the edges are 

 sharp. The outer edge is flat and the inner edge sharp, the whole effect 

 being that of a forged knife blade at each end. One end of such a fiber 

 is seen in Figure 96. 



The body of the fiber is composed of a very coarsely granular sarco- 

 plasm, containing a great many thick myo-fibrils that run in slightly 

 curving lines from end to end. The fibrils are thickest near the surfaces, 

 and only in the interior of the fiber is it possible to see the sarcoplasm. 

 It occurs here in several long, spindle-shaped masses, in the largest of 

 which lies the nucleus. This nucleus is oval and not specialized in any 

 particular way. It is not even much elongated, as many such nuclei 

 would be under similar conditions. 



