THE PARENCHYMAL MUSCLES. 



291 



The Muscular System. — The fibres of which this consists are some- 

 times separate, sometimes united, but in the adult stages run so 

 regularly in the three dimensions of space, that one can fitly designate 

 them longitudinal, transverse, and sagittal muscles. As in other 

 animals without a skeleton adapted for locomotion, these muscles are 

 wholly composed of smooth or so-called " organic " fibres. One looks 

 indeed in vain for a nucleus, at least in those which are full-grown. 

 They consist of a perfectly homogeneous, strongly refractive proto- 

 plasm, showing at most, and that only in the thick fibres, a differen- 

 tiation into peripheral and axial substance. The ends are narrowed 

 and drawn out into more or less long 

 threads, not unfrequently dichotomising. 

 On the isolated fibres one often sees late- 

 ral processes, as thin, delicate filaments, 

 forming sometimes a distinct network. 



One may even hazard the supposition that 



the intercellular network above described 



was of a muscular nature. 



These parenchymal muscles attain 



their greatest development in the musculi 



longitudinales which occur in all Cestodes, 



and which run, especially down the inner 



part of the cortical layer, in the form of 



numerous strong bands (Fig. 147). This 



is most distinctly and most beautifully 



seen in the younger joints, where the 



fibres are arranged in closer groups, 



though they are of course much shorter 



than is afterwards the case. These deeper 



bands exhibit the thickest fibres — the 



thickest, indeed, in the whole body. Ex- 

 ternally both bundles and fibres thin off, 



sometimes gradually, sometimes suddenly, 



until in the close neighbourhood of the 



cuticle we only find the fine isolated 



fibres. In many cases these can be followed into the longitudinal 



fibres of the musculo-dermal sheath, so that the distinction between 



the two systems becomes somewhat fictitious. 



Like the longitudinal muscles, those which run transversely are 



usually well developed, and grouped in bands of considerable 



strength. This is at least true of the main mass of those fibres, 



which bound the middle layer, and which (except in Twnia undulata 



and a few others) ^^■,^'E§^bY^^O§Sft<^ continuous layer, lying 



Fig. 147. — Longitudinal section 

 of Tcenia saginata (young chain of 

 joints). { X 25.) 



