THE FIBER CELLS. 235 



At the time of hatching, it is very voluminous, and is apparently confined to the 

 thorax. (Fig. 151, in.t.) In the later stages, it undergoes considerable reduction, 

 and its posterior end becomes attached to the anterior margin of the abdominal 

 shield. (Fig. 77, in.t.) 



The other large muscle developed from the fiber cells is the anterior end 

 of the hypobranchial. (Fig. 77, B.b.th.m.) This muscle also becomes greatly 

 reduced in volume in the older stages, and its anterior end retreats to a more 

 posterior position. 



Many other fiber cells become scattered irregularly along the margins of the 

 thoracic shield, in the spaces left free by the contracting liver lobes. Here they 

 elongate and form the small, scattered bundles of muscle fibers that permanently 

 unite the neural and haemal walls of the cephalic buckler. 



The metamorphosis of fiber cells into these particular muscles on the haemal 

 surface of the thorax, takes place at a very late period, long after the endocranium, 

 the haemo-neural, and other muscles have become clearly differentiated; and 

 no other muscles than those mentioned are formed in this manner. At the 

 time the fiber cells undergo their metamorphosis, the yolk lobes are invested with 

 a thin cellular layer, so that it is hardly probable that any of them are absorbed in 

 the yolk with the haemal blastoderm. 



A large number of fiber cells, however, never form definite muscles. They 

 persist through life, loosely distributed throughout the lacuna spaces in all parts 

 of the body, although they appear to be more numerous at the base of the append- 

 ages and in the anterior portions of the cephalic shield. 



In the half grown Limuli, and even in the adult, they may be readily recognized 

 by their large size, peculiar structure, and coloring. They generally preserve 

 the spindle-shaped form, but appear to be somewhat amoeboid, or rather euglenoid, 

 and some have been observed in division. (Fig. 131, e.) The fiber is now 

 much less refractive and has lost its distinctly spiral arrangement, apprearing as 

 longitudinal striae that converge toward either end. 



The remarkable history of the fiber cells indicates that they are the degenerat- 

 ing remnants of the longitudinal haemal muscles of the first five thoracic segments. 

 Only a part of the large number of cells actually form muscles; the others become 

 free cells that may be regarded as a special type of blood corpuscle, although they 

 do not appear to circulate freely in the main blood channels. 



In the spiders and in the scorpion, the same kind of fiber cells are present. 

 Juts what their history is in these forms has not been definitely determined. Vari- 

 ous authors, who have apparently seen them in the spiders, state that they give 

 rise to blood corpuscles. 



Whether these cells ultimately give rise to true blood corpuscles or not 

 was not determined. 



Blood corpuscles of the usual type arise from the peripheral ends of the lateral 

 plates of the thoracic and abdominal segments, and at these early stages can be 

 readily distinguished from the fiber cells. (Fig. 131, b.c.) 



