66 



INVERTEBRATE ZOOLOGY 



Striated muscles occur even in forms as low as the medusae 

 (Fig. 42), where they remain on the surface of the body. They 

 usually originate as epithelial cells. The nuclei of cells which are 

 to form striated muscle undergo a series of divisions without sub- 

 sequent division of the cytoplasm. In the cytoplasm of the poly- 

 nuclear cells thus formed, numerous fibrils make their appearance. 

 The fibers are surrounded by a sarcoplasm containing the nuclei, 

 and the whole muscle element is covered with a sheath which is 

 termed the sarcolemma. Each fibril is composed of two different 

 kinds of substance, one called the isotropic substance which 

 does not stain readily, and another, the anisotropic substance 

 which is doubly refractive and stains deeply. The two sub- 

 stances alternate regularly along the fibril, and in adjoining 

 fibrils they are in alignment so that in stained 

 preparations under the microscope a bundle of 

 ^IfJm^. fibrils has a cross-striped appearance. Peculiar 

 groupings of fibrils occur in some muscle cells 

 (Fig. 43). 



Nervous Tissue 



Irritability, or the power of reacting to stimuli, 

 is inherent in all protoplasm but even in the single- 

 FiG 43 — celled Protozoa it has been shown that parts of the 

 Muscle fiber cytoplasm become specialized as a neuromotor 



from adductor , o u • t a- u u i 



apparatus, feuch specialization has been demon- 

 strated for some of the ciliates in which it seems 

 probable that stimulation is propagated along 

 certain tracts or fibrils more efficiently than 

 through undifferentiated cytoplasm. In most 

 Metazoa, a nervous system is developed for transmitting 

 sensory and motor impulses through the linking together of 

 highly specialized nerve cells or neurons. Nervous impulses 

 are propagated through a plexus of scattered cells in the hydroid 

 stage of the coelenterates. 



Characteristically, a nerve cell consists of a nucleated cell 

 body of cytoplasm from which one or more cytoplasmic processes 

 are given off. There is a definite polarity in nerve cells. A 

 given fibril propagates an impulse in only one direction. When 

 there are several cytoplasmic processes from the cell body of a 

 neuron those which transmit the impulse inward toward the cell 

 body are termed dendrites. Normally, but a single process 



muscle of mus- 

 sel, Anodonta 

 mutabilis. 

 (After Schnei- 

 der). 



