NERVOUS TISSUE. 39 



cells which form the outer and inner layers are prolonged into contractile 

 roots. Here then we have cells of which a special part discharges a 

 contractile or muscular function, while the other parts retain other 

 powers. 



In other Ccelentera the muscular cells are still directly connected with 

 the epithelium, but become more and more exclusively contractile. In 

 all other animals the muscular tissue is derived from the mesoderm, 

 which, as we have already mentioned, is not distinctly present in 

 Ccelentera. In the majority, the muscle cells arise on the walls of the 

 body cavity, and their origin may often at least be described as epithelial. 

 But in other cases the muscles arise from those wandering "mesenchyme" 

 cells to which we have already referred. 



Structure. — A distinction is usually drawn between striped and un- 

 striped muscle fibres, but the distinction seems to be one of degree. 



Smooth or unstriped muscle fibres are elongated contractile cells, 

 externally homogeneous in appearance. They are especially abundant 

 in sluggish animals, e.g.. Molluscs, and occur in the walls of the gut, 

 bladder, and blood vessels of Vertebrates. They are less perfectly 

 differentiated than striped muscle fibres, and usually contract more 

 slowly. 



A striped muscle fibre is a cell, the greater part of which is modified 

 into a set of parallel longitudinal fibrils, with alternating " clear and 

 dark " transverse stripes. Each fibril has certainly a complex internal 

 structure, but according to Haycraft the "stripes " are the optical effects 

 of the ampuUated or beaded form of the fibrils, and can be seen even on 

 a collodion film, upon which the muscle fibres are pressed. A residue of 

 unmodified cell substance, w4th a nucleus or with many, is often to be 

 observed on the side of the fibre, and a slight sheath or sarcolemma 

 forms the "cell wall." Many muscle fibres closely combined, and 

 wrapped in a sheath of connective tissue, form a muscle, which, as every 

 one knows, can contract with extreme rapidity when stimulated by a 

 nervous impulse. 



(d) Nervous Tissue. 



Origin. — Beginning again with the Amccba, we recognise that it is 

 diffusely sensitive, and that a stimulus can pass from one part of the cell 

 to another. 



In some Ccelentera some of the external cells seem to combine con- 

 tractile and nervous functions. Therefore they are sometimes called 

 " neuro-muscular." 



But in Hydra there are special nervous cells, whose basal prolonga- 

 tions are connected with the contractile roots already described. This 

 is a neuro-muscular apparatus of the simplest kind. The nerve cells 

 probably receive impressions from without, and transmit them as stimuli 

 to the contractile elements. 



In sea anemones and some other Crelentera, there is an interesting 

 complication, withal very simple. There are superficial sensory cells, 

 connected with subjacent nerve or ganglion cells, from which fibres pass 

 to the contractile elements. 



In higher animals the sensory cells are integrated into sense organs, 

 the ganglionic cells into ganglia, while the delicate fibres which form 



