104 



GEODIA MESOTRIAENA. 



and in var. microana (Plate 24, figs. 2a, 6a) most of these rhabds are quite or 

 nearly isoactine amphioxes, usually nearly cylindrical in the middle, and abruptly 

 attenuated towards the blunt ends. A few are distinctly anisoactine and 

 rounded at the thicker end so that they appear as styles. In var. pachana 

 (Plate 24, figs. 3a, 5a) these spicules are more spindle shaped and usually taper 

 more gradually towards the ends. It is also to be noted that strongly anisoac- 

 tine amphioxes and true styles are much more numerous among these spicules 

 in var. pachana than in the other two varieties. 



DIMENSIONS OF SMALL DERMAL RHABDS OF GEODIA MESOTRIAENA. 





Var. pachana 



Var. megana 



Var. microana 



Length mm 



420-680 



335-571 



380-550 



Thickness /t 



10-19 



10-18 



9-15 



The small amphioxes of the interior of the choanosome are similar to those 

 in the tufts but somewhat smaller. In var. megana they measure 170-440 

 in length and 7-17 jx in thickness. As stated above these spicules may be young 

 stages of the small superficial rhabds in the tufts. 



The rhabdomcs of the ortho- and plagio-triaenes (Plate 21, figs. 3-5; Plate 

 23, figs. 16, 20-23) are nearly straight or curved, more or less cylindrical in their 

 cladomal part, and conical in their acladomal part. The acladomal end is sharp 

 pointed or blunt. The observation of thin transverse splinters lying flat, with 

 the rhabdome-axis vertical, shows the axial rod of the rhabdome to be triangular 

 in transverse section. Usually this rod (the canal wherein it lies) is quite nar- 

 row, 1 n broad or less. In some of these spicules, however, I found, after boiling 

 them in nitric acid, the axial canal in the acladomal part of the rhabdome as 

 much as 9 /i in diameter and wide open at the end, the latter having the shape of 

 a very thin-walled tube. The rhabdome is 4.6-7.2 mm. long and at the cladomal 

 end 75-120 fi thick. The clades are 200-670 /i long; their chords enclose angles 

 of 85-117°, on an average 98.5°, with the axis of the rhabdome. Very often 

 the three clades of the same cladome differ considerably in size; ortho- and 

 plagio-triaenes showing such an irregularity appear in fact to be more frequent 

 than the regular ones. At the base the clades are always curved, concave 

 towards the rhabdome; their distal part is generally straight (Plate 24, figs. 16, 

 21, 22), or slightly curved in the opposite direction (Plate 24, fig. 20), more 

 rarely abruptly bent towards the rhabdome (Plate 24, fig. 23). In var. megana 

 the clade-rhabdome angle of these spicules is on an average only 91.9°, while it 



