MUSCLE TISSUE 



47 



changing them into sensations and carrying on the stimulus to the 

 muscular elements. 



In accordance with this, muscle tissue in its simplest form appears 

 as a system of epithelial muscle cells (Fig. 47, a). They still lie in the 

 epithelium, but no longer take part in limiting the body surface. 

 They form inwards muscle processes which run under the epithelium. 

 They are distinguished from neuro-muscular cells by the fact that their 

 chief function is contractility, and that the body of the cell no longer 

 acts as a neural portion. This cell body then appears merely a remainder 

 of the original formative substance of the fibres ; it lies below the 

 upper epithelial cells, wedged in between them. In this way a neuro- 



FiG. 47. — Muscle fibres, u, Epithelial muscle cell (ez\ witli fibre ; 6, siib-epithelial muscle fibre, 

 with attached protoplasmic body, both of Cnidaria ; c, longitudinal section of muscle fibre of a 

 Nematode ; Ci, transverse section of the same ; e, of an Hirudo ; ei, the same in transverse section ; 

 d, dorso-ventral muscle fibre of a marine Planarian ; /, the same of an Hirudo ; g, branched muscle 

 fibre from the jelly of a Cienophove ; y>, protoplasm, or, in e and e^, medulla ; cs, contractile 

 substance ; %, nucleus. 



muscular cell becomes an epithelial muscle fibre, with a protoplasmic 

 body containing a nucleus attached to it. From this to sub-epithelial 

 muscle fibres (Fig. 47, V) is a short step. These no longer lie in, but 

 under the epithelium. They consist of a contractile fibre, which 

 carries along its whole length, on the side bordering on the epithelium, 

 a thin layer of protoplasm. 



In other cases, e.g. in the Ctenopliora, epithelial cells can leave their 

 complex and enter into the jelly secreted below the epithelium. Here 

 they often grow into fibres branched at each end (Fig. 47, g), whose 

 protoplasm changes into a contractile substance. Such fibres are 

 known as mesenehymatous muscle cells, as opposed to epithelial 

 muscle cells. 



