316 HYALONEMA (OONEMA) BIANCHORATUM PINULINA. 
The relative thickness (thickness x 10000:length) of the superficial 
(dermal and gastral) amphioxes of different lengths of both specimens together 
is shown in the following table: — 
O:41— | (0,61 avers 
0 
4 
Length, ; 
ength, mm 6 08 
ee 
aot 
CO & 
1.81-2 
bo 
Relative thick- | limits | 177-356 | 109-447 | 09-39 | 075-445 | 084-346 | 08-393 | 082-159 | 074-147 
ness (thickness | | 
x 10000: ——— 
length) | 
average | 279.3 | 309.3 | 231.7 217.3 156.2 | 202.4 | 118.7 101.5 
This correlation between length and relative thickness comes out very clearly 
in the curve -------- in the graph., Fig. 17. 
To ascertain whether the superficial amphioxes of varying relative thickness 
form a continuous series I arranged the 94 measured according to this relation 
and the result is represented in Figure 18. 
The frequency-curve of the different relative thicknesses in this figure 
clearly shows that, biometrically, two kinds of superficial amphioxes must be 
distinguished: — more slender ones, in which the proportion of length to thick- 
ness is 10000 : 74-210, most frequently about 10000 :112, and stouter ones 
in which this proportion is 10000 : 210-447, most frequently about 10000 : 
312. Among the superficial amphioxes 
0.41-0.8 mm. long 82 % belong tothe stout and 18% to the slender kind. 
ORT ce cc 49 % (a OG % a (a 51% Ce he 6c 66 
116 cc a D7 % 6c (She 4h (ats 6c 73% (te 5 a a 
A (a3 (3 0 % (a3 GS 15 (a3 (a3 100% i 35 “ 66 
It is to be noted that the shortest superficial amphioxes observed, that is 
those 0.41—0.6 mm. long, are on the whole relatively not quite so stout as those 
0.61-0.8 mm. long, which are on the whole the stoutest. These smallest am- 
phioxes are probably young forms. Also among the spicules 0.61-1.6 mm. 
long there are, no doubt, some young forms of larger ones. And since the 
larger ones are all the slender kind these young forms will probably for the most 
part be young forms of the slender ones. 
The distal ends of the spicules of the free part of the stalk are all broken off. 
The parts present have a maximum thickness in specimen a of 0.8 mm., in b 
of 1.3mm. The spiral twist, which all the long fragments exhibit, is such that 
there is one whole turn in about 23 em. of length. The spicules forming the 
