280 HYALONEMA (PRIONEMA) CRASSUM. 
The hexactine megascleres (Plate 108, figs. 10-13) are 0.7-2.5 uw in diameter, 
and have straight or slightly curved, blunt, conical rays 20-55 u thick at the base. 
Three kinds of amphioxes can be distinguished: — large ones, confined to the 
choanosome; intermediate and smail 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 » thick. These spicules are 90-140 times as long as thick. 
The intermediate and small slender amphioxes of all parts of the body are 
0.67—2.5 mm. long and 10-35 u 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 » long and 13-50 » thick. These amphioxes are 17-39 times as long 
as thick. A good many of them are distinctly centrotyle, the central tyle being 
sometimes 9 » 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 u in length are less than 40 times as long as thick, while all those 
over 940 u in length measured are, with a single exception, more than 40 times 
as long as thick. 
The microhexactines (Plate 106, figs. 4-12, 31-37) are 108-175 u in diameter, 
on an average about 140 4. 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 » thick, usually 5-6.5 uw, and 
spined. The spines of the proximal part of the ray are sparse, vertical, broad 
conical, sharp-pointed, and 1.5-2 4 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 amphidises 
