Metazoa Rana. 273 



The alimentary canal, therefore, consists of a tube for 

 conveying the food to a storing and digesting organ, from 

 which it is passed into another tube, in which it is mixed 

 with and further acted on by digestive fluids secreted by 

 certain organs opening into the canal, through whose walls 

 absorption of nutriment takes place. The refuse is col- 

 lected in the rectum and ejected to the exterior periodically. 

 The wall of the oesophagus is composed of three layers, 

 internally (i) mucous membrane with mucous glands (fig. 

 132, d, e, f, g) surrounded by (2) a layer of muscle fibres 

 (, c) and (3) of connective tissue (a). It will be most con- 

 venient to refer briefly at this point to the essential charac- 

 ters of the muscular and connective tissues, since we shall 

 meet with them in almost every part of the body. 



Muscle. Like every tissue in the organism muscle is 

 composed of cells. The cells are much elongated in one 

 direction, and have the function of contractility specially 

 developed. There are two varieties of muscle, striped and 

 non-striped. The protoplasm of the cell or fibre of striped 

 muscle is greatly modified, being transformed into a bundle 

 of exceedingly delicate fibrillae, each of which appears to 

 consist of alternate dark and light segments. The cell-wall 

 is exceedingly thin, and is known as the sarcolemma. 

 Nuclei may be observed scattered here and there through 

 the fibre, or immediately below the sarcolemma of the 

 muscle-fibres of higher forms. The alternate light and dark 

 segments of the individual fibrillae give the fibre as a whole 

 a transversely striped appearance, whence this variety of 

 muscle derives its name. Striped muscle is also known as 

 * voluntary ' because it is the variety met with in muscles 

 under the direct command of the will. There are, however, 

 exceptions to this rule, as, for instance, in the heart, where 

 the fibres are striped, but where the muscles are independent 

 of the influence of the will. The voluntary muscles of the 

 frog are sometimes branched as in the tongue (fig. 135). 



The other variety of muscle, termed non-striped or 



T 



