696 LIFE : OUTLINES OF GENERAL BIOLOGY 



MUSCULAR TISSUE— An Amoeba is a diffusely contractile single 

 Cell, without any specialised motor structures, but in many Proto- 

 zoa, as in the stalk of the Bell Animalcule (Vorticella), there is the 

 beginning of differentiation in the form of threads or myonemes, 

 and the very frequent cilia and flagella are other familiar illustra- 

 tions. In a ciUum the protoplasmic thread is alternately flexed and 

 straightened again; in a flagellum there is an undulatory movement 

 along the length of the thread or lash. 



In sponges there is incipient muscular tissue, taking the form of 

 rings or rows of spindle-shaped cells; but most of the contractile 

 activity of the sponge is in the flagella of the internal cells that 

 keep up the inhalant and exhalant currents of the characteristic 

 canal system. 



In Hydra and some other simple Coelentera the bases of some of 

 the epithelial cells of the outer and inner laj'crs (ectoderm and 

 endoderm) are prolonged into contractile roots. This is an interesting 

 division of labour, for the contractility is mainly restricted to the 

 basal processes. A somewhat similar primitiveness is seen in the 

 muscle cells of threadworms (Nematodes), where the inward-pro- 

 jecting portions are as markedly non-contractile as the outer por- 

 tions are contractile. The line of evolution has been the reduction 

 of the non-contractile portion of the muscle cell, and many interest- 

 ing gradations can be seen within the groups of Coelentera and worms. 

 But from the level of certain Coelentera, e.g. jellyfishes, onwards, 

 there are specialised muscle fibres, a fibre being either a much 

 elongated single cell or a union of several elongated cells in one line. 



In sluggish animals, such as tapeworms and tunicatcs, the mus- 

 cular elements are smooth and unstriated muscle cells. Each is an 

 elongated spindle with a central nucleus, and with a longitudinal 

 intracellular fibrillation. These unstriped muscle cells often occur 

 in long rows, the component units very closely apposed. Compared 

 with ordinary striped muscle cells, they arc less differentiated, and 

 they contract more slowly. In sluggish animals like molluscs there 

 is much unstriped muscle, but those muscles that do something 

 quickly, such as shutting the shell-valves of a scallop, have striped 

 fibres or fibres with unstriped fibrils twisted in a spiral. In the 

 higher backboned animals, such as mammals, the unstriped muscles 

 are practically restricted to {a) the wall of the food-canal (effecting 

 the slow peristaltic movements which press the food onwards), 

 (b) the wall of the bladder (effecting the slow contractions in urina- 

 nation), and (c) the walls of the arteries. It is interesting to note 

 that in the archaic Invertebrate Peripatus, all the muscles of the 

 body are unstriped, save those that work the jaws! 



In a striped muscle fibre the greater part of the cell has gone to 

 form a set of parallel longitudinal fibrils, with alternating "clear 

 and dark" transverse striae, whose details are somewhat compli- 



