BY E. F. KALLMANN. 345 



thin stained section) gradually assumes towards its exterior a 

 structure somewhat resembling that of a stratified epithelium. 



Spicules. — {a) The prevailing megasclere — that participating 

 in the formation of the fibres — is a symmetrically curved, slightly 

 fusiform, irregularly ended amphioxea, varying in length from 

 about 240 /x (in the case of the slenderest) to slightly more than 

 500/x, and in diameter from (seldom less than) 13 up to about 28 /x. 

 The curvature when most pronounced is usually somewhat angu- 

 late. Except in the case of the slenderer (? immature) indi- 

 viduals, which for the most part (or perhaps exclusively) occur 

 only between the fibres, the spicule narrows to its extremities as 

 a rule, not by a continuous gradual tapering but by a series of 

 more or less abrupt contractions that commence not farther than 

 30 /x from the extremities. The endmost contraction is frequently 

 very pronounced, and the spicule is thereby rendered apicu- 

 late ; the terminal portion of the spicule is then either sharp- 

 pointed, resembling a mucro, or is rounded off at the point 

 and nipple-shaped. A small proportion of the spicules are in- 

 termediate in the form of their extremities between oxea and 

 strongyla, and rare styli also occur, the form of which clearly 

 shows them to be the result of failure on the part of one of the 

 actines of the oxea to attain to its normal development. In 

 addition, there are present exceedingly scarce (apparently) ab- 

 normal, forms of cylindrical shape, either symmetrically ended 

 (strongyla) or with one extremity abruptly narrowed, which 

 range in length from less than 60 to (very rarely) upwards of 

 180 /x, and in stoutness from 8 to 14/x; they recall the some- 

 what similar spicules of Gellius rhaphidophora, and should perhaps 

 be reckoned as constituting a form distinct from the above 

 spicules. 



(b) The second form of megasclere is comparatively rare, and 

 occurs scattered. Like the preceding, it is diactinal and very 

 often exhibits some degree of irregularity in the formation of its 

 extremities; but it differs in being of greater length and rela- 

 tively slenderer, in having always more or less rounded ex- 

 tremities, and in being as a rule without curvature. The length, 

 seldom if ever less than 500 /x, may attain to 810 /ix; and the 



