BY E. P. KALLMANN. 285 



occur also, abundantly, disposed concentrically around the centre 

 from which the fibres radiate, forming a well-marked spherical 

 " nucleus " to the sponge. Between the fibres elsewhere, mega- 

 scleres are rare or absent. The spherasters are found chiefly in 

 the outer region or the cortex, and in the peripheral layer of the 

 choanosome close beneath the cortex; they are provided with, 

 usually, from 11 to 15 actually countable rays, and vary in their 

 maximum total diameter, in diff'erent specimens, from about 50 to 

 90 /x. It appears to be the rule that, in specimens in which the 

 maximum diameter of the spherasters is less than 70 //, the rays, 

 for the most part, are shorter than the diameter of the centrum, 

 and frequently are bifurcate at the extremity; whereas when the 

 spicule is of greater diameter than 70 /x, the rays appear usually 

 to be longer than the diameter of the centrum and to be only 

 very rarely forked. The chiasters (which are almost entirely 

 confined to the choanosome) are sometimes abundant, sometimes 

 rather scarce; they usually have from 6 to 10 rays, the surface of 

 which is minutely tuberculate. The diameter of the chiasters that 

 occur in the cortex rarely exceeds 12 or 13/x, while those within 

 the choanosome range in diameter up to 18 or 20 /x; also, in the 

 case of the smaller chiasters, whether in the cortex or the choano- 

 some, the rays usually are slightly expanded at the tip, whereas 

 the larger ones approach more closely in form to oxyasters, and, 

 in addition, they occasionally exhibit a branching of their rays; 

 there would thus appear to be an incipient differentiation of the 

 chiasters into two forms, tyl asters and oxyasters. As inter- 

 mediates between the chiasters and the microstrongyles, some- 

 what plesiaster-like forms are commonly met with, in which the 

 rays proceed, not from a common centre, but from a shorter or 

 longer axis, and are usually also reduced in number. In addition 

 to these, triradiate or Y-shaped forms are frequent, as well as 

 bent rods derived from the latter through the loss of one ray. 

 The microstrongyles occur in moderate abundance throughout 

 the entire cortex and are densely aggregated to form a thin layer 

 immediately below the surface; they are also scattered through 

 the choanosome, gradually decreasing in numbers towards the 

 centre of the sponge. They vary from 6 to (rarely) 20 /x in 



