280 HYALONEMA (PRIONEMA) CRASSUM. 



• The hexadine megasderes (Plate 108, figs. 10-13) are 0.7-2.5 ^ in diameter, 

 and have straight or slightly curved, blunt, conical rays 20-55 m thick at the base. 



Three kinds of amphioxes can be distinguished : — large ones, confined to the 

 choanosome; intermediate and small ones, found both radially situated in the 

 choanosome and paratangentially situated in the dermal and gastral membranes ; 

 and small stout paratangentially situated ones, confined to the dermal and 

 gastral membranes. 



The large amphioxes of the choanosome measured are 2.5-8.5 mm. long and 

 18-80 n thick. These spicules are 90-140 times as long as thick. 



The intermediate and sryiall slender amphioxes of all parts of the body are 

 0.67-2.5 mm. long and 10-35 m thick. These spicules are 40-160 times as long as 

 thick. 



The small stout amphioxes [tignules) of the dermal and gastral membranes 

 are 410-980 yu long and 13-50 n thick. These amphioxes are 17-39 times as long 

 as thick. A good many of them are distinctly centre tyle, the central tyle being 

 sometimes 9 ti more than the adjacent parts of the spicule in transverse diameter. 

 The gastral small stout amphioxes (Plate 107, fig. 11) are, on the whole, 

 relatively considerably thicker than the dermal, the former being on an average 

 23.7 times, the latter 28.4 times as long as thick. 



Taking all the amphioxes of the sponge together we find that all those 

 under 650 ix in length are less than 40 times as long as thick, while all those 

 over 940 ix in length measured are, with a single exception, more than 40 times 

 as long as thick. 



TYiQ microhexadines (Plate 106, figs. 4-12, 31-37) are 108-175 /^ in diameter, 

 on an average about 140 n. The rays of the same spicule are generally equal 

 and regularly arranged, but are exceptionally unequal in length. The rays are 

 straight, conical, sharp-pointed, at the base 4.5-7 n thick, usually 5-6.5 n, and 

 spined. The spines of the proximal part of the ray are sparse, vertical, broad 

 conical, sharp-pointed, and 1.5-2 ^ high. Distally the spines become more 

 numerous and crowded, smaller, more slender, and more and more inclined back- 

 wards towards the centre of the spicule. 



From the morphological point of view four kinds of amphidiscs can be dis- 

 tinguished: — 1, large ones with thick shaft, broad and short anchors, and 

 smooth teeth; 2, large ones with slender shaft, medium anchors, and serrated 

 teeth; 3, small ones with long and slender anchors; and 4, still smaller ones with 

 medium anchors. 



Examined biometrically, according to their length frequency, the amphidiscs 



