84 



GEODIA ATAXASTRA. 



Although somewhat irregular, both curves have one very well-pronounced 

 main culmination, broad in the var. latana curve, and narrow in the var. angus- 

 tana curve. These main culminations show that one amphiox-length is much 

 more frequent than any other. The most frequent lengths represented by these 

 culminations are 1.4 to 1.5 mm. in var. latana and 1.8 mm. in var. angustana. 

 These may be considered as the normal amphiox-lengths. That the "normal" 

 amphiox-length is greater in var. angustana than in var. latana accords with the 

 fact that the specimen of var. angustana selected for this examination is much 

 larger than the specimen of var. latana. 



The var. latana curve has one, the var. angustana curve two, minor culmi- 

 nating points besides the main one. The left one (at 1.3 mm.) of the latter 

 appears to correspond to the single one (at 0.9 mm.) of the former. Since the 

 majority of amphioxes shorter than those of "normal" length (1.4-1.5 mm. in 

 var. latana and 1.8 mm. in var. angustana) are probably young, still growing 

 spicules, the minor culmination which precedes the main one in both curves 

 indicates that the rate of longitudinal growth of these spicules is not uniform, 

 a stage of cessation or retardation of growth at 0.9 mm. in var. latana and at 

 1.3 mm. in yar. angustana intervening between the earlier and the later periods 

 of rapid growth. 



The rare large am/phistrongyles have similar dimensions to the large 

 amphioxes. 



The rare large styles are 0.5-1.6 mm. long and 11-30 /x thick. Their thick- 

 ness is by no means proportional to their length. 



The rare angularly bent and irregularly branched am-phiox-derivates have 

 similar dimensions as the regular amphioxes. In the angularly bent amphiox- 

 derivates the bend is always near one end. The angle may be over or under 

 90°. Most of the branched forms are amphiclade in character and consist of a 

 shaft with a short branch-ray near each end. In some only one branch-ray, 

 situated near one end of the shaft, is observed. Generally the branch-rays are 

 simple, straight, conical, and pointed, rarely irregularly curved or divided into 

 secondary branchlets. The simple branch-rays are 20-100 p. long and rise 

 vertically or obliquely from the shaft. The oblique ones are inclined outward, 

 proclade-fashion. 



The minute rhabds of the distal part of the choanosome, the cortex, and the 

 dermal groups (Plate 44, figs. 31a, 32, 33a, 40a) are, in var. angustana, mostly 

 fairly straight, rather abruptly pointed amphioxes. In var. latana minute 

 dermal rhabds rounded at one end or at both occur in fair numbers besides the 



