Ixiv THE VOYAGE OF H.M.S. CHALLENGES. 



between the radial bars, singly or in groups ; sometimes their number seems to be not 

 greater than that of the bars, whilst in other cases a circlet or group of axopodia 

 corresponds to each radial bar. Perhaps their fine axial thread consists of acanthin. At 

 all events the axopodia are constant organs (probably sensory, like the " palpocils ") 

 and not retractile like the movable myxopodia. 



The axial threads in the pseudopodia of the Acanthometra were first discovered by 

 E. Hertwig, who accurately described their peculiar structure and arrangement (L. N. 33, pp. 16, 117). 



96. The Myophriscs of the Acanthometra. The Acanthometra are charac- 

 terised by a very peculiar differentiation of the exoplasm, namely, by the formation of 

 myophriscs or contractile threads from the sarcodictyum. In most (and perhaps in 

 all) ACANTHARIA of this order each radial bar is surrounded by a circlet of such 

 contractile threads, which was first described as a " ciliary corona " (see note A, below). 

 The number of contractile threads in each circlet usually amounts to from ten to 

 twenty, rarely being more than thirty and less than eight ; it often appears to be 

 constant in the individual species (see note B). In the living state the myophriscs are 

 long, thin filaments, the pointed distal end of which is inserted into the radial bar, 

 whilst the thicker proximal end is attached to the surface of the calymma, which 

 is elevated round the base of each rod into the form of a gelatinous cone or skeletal 

 sheath (see note C). Probably the myophriscs lie on the outer surface of the apical 

 portion of this gelatinous cone, and are hence to be regarded as exoplasmic threads 

 differentiated from the sarcodictyum. Sometimes, however (as in Acanthochiasma), 

 they fuse into a contractile membrane and form the envelope of a cone, whose 

 interior is occupied by a gelatinous papilla of the calymma. On mechanical irritation 

 the myophriscs contract rapidly and suddenly, like muscle-fibrillse, becoming at the 

 same time thicker, and hence are very different from pseudopodia. Their distal point 

 of insertion being fixed to the firm acanthin rod, they raise by their contraction the 

 skeletal sheath, to which their bases are attached or in the surface of which they lie. 

 The result of their contraction is therefore a distention and increase in volume of the 

 calymma, with which is no doubt connected an inception of water into the gelatinous 

 mass, and hence a diminution in its specific gravity. Probably the Acantho- 

 metra contract their myophriscs voluntarily when they wish to rise in the water ; 

 when these relax the calymma collapses owing to its elasticity, water is then expelled 

 and the specific gravity increases. From a physiological point of view, then, the 

 myophriscs are to be regarded as a hydrostatic apparatus, morphologically as myo- 

 phanes or muscular fibrillse, such as also occur in the intracapsular protoplasm 

 (see 7780). On more violent irritation and after the death of the Acantho- 

 metra the myophriscs separate from the radial bars and remain attached to the distal 

 ends of the conical gelatinous sheaths as free " ciliary coronas." At the same time, 



